Multiplexing of real time services and non-real time services for OFDM systems

ABSTRACT

For certain embodiments of the present disclosure, a method and apparatus for transmitting in a multi-antenna communication system is provided. The method comprises modulating a plurality of signals to be transmitted from each antenna of a plurality of antennas with at least one subband of a different one of a plurality of groups of subbands, wherein each of the plurality of groups includes a different subset of the plurality of subbands and wherein the plurality of subbands of a first group are noncontiguous, and transmitting at least some the plurality of signals substantially simultaneously from different ones of the plurality of antennas.

This application is a continuation of U.S. patent application Ser. No.11/165,451 filed Jun. 23, 2005 entitled “Multiplexing of Real TimeServices and Non-Real Time Services for OFDM Systems” which is acontinuation of application Ser. No. 09/614,970, Jul. 12, 2000, now U.S.Pat. No. 6,952,454 which is a continuation-in-part of U.S. patentapplication Ser. No. 09/532,492, entitled “HIGH EFFICIENCY, HIGHPERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING MULTI-CARRIER MODULATION,”filed Mar. 22, 2000, now abandoned, and U.S. patent application Ser. No.09/539,224, entitled “METHOD AND APPARATUS FOR MEASURING REPORTINGCHANNEL STATE INFORMATION IN A HIGH EFFICIENCY, HIGH PERFORMANCECOMMUNICATIONS SYSTEM,” filed Mar. 30, 2000, now U.S. Pat. No. 6,473,467issued on Oct. 29, 2002, both of which are incorporated herein byreference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to data communication. More particularly,the present invention relates to a novel and improved communicationssystem employing multi-carrier modulation and having high efficiency,improved performance, and enhanced flexibility.

II. Description of the Related Art

A modern day communications system is required to support a variety ofapplications. One such communications system is a code division multipleaccess (CDMA) system that conforms to the “TIA/EIA/IS-95 MobileStation-Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System,” hereinafter referred to as the IS-95standard. The CDMA system supports voice and data communication betweenusers over a terrestrial link. The use of CDMA techniques in a multipleaccess communication system is disclosed in U.S. Pat. No. 4,901,307,entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USINGSATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459,entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULARTELEPHONE SYSTEM,” both assigned to the assignee of the presentinvention and incorporated herein by reference.

An IS-95 compliant CDMA system is capable of supporting voice and dataservices over the forward and reverse communications links. Typically,each voice call or each traffic data transmission is assigned adedicated channel having a variable but limited data rate. In accordancewith the IS-95 standard, the traffic or voice data is partitioned intocode channel frames that are 20 msec in duration with data rates as highas 14.4 Kbps. The frames are then transmitted over the assigned channel.A method for transmitting traffic data in code channel frames of fixedsize is described in U.S. Pat. No. 5,504,773, entitled “METHOD ANDAPPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to theassignee of the present invention and incorporated herein by reference.

A number of significant differences exist between the characteristicsand requirements of voice and data services. One such difference is thefact that voice services impose stringent and fixed delay requirementswhereas data services can usually tolerate variable amounts of delay.The overall one-way delay of speech frames is typically required to beless than 100 msec. In contrast, the delay for data frames is typicallya variable parameter that can be advantageously used to optimize theoverall efficiency of the data communications system.

The higher tolerance to delay allows traffic data to be aggregated andtransmitted in bursts, which can provide a higher level of efficiencyand performance. For example, data frames may employ more efficienterror correcting coding techniques requiring longer delays that cannotbe tolerated by voice frames. In contrast, voice frames may be limitedto the use of less efficient coding techniques having shorter delays.

Another significant difference between voice and data services is thatthe former typically requires a fixed and common grade of service (GOS)for all users, which is usually not required or implemented for thelatter. For digital communications systems providing voice services,this typically translates into a fixed and equal transmission rate forall users and a maximum tolerable value for the error rate of speechframes. In contrast, for data services, the GOS may be different fromuser to user and is also typically a parameter that can beadvantageously optimized to increase the overall efficiency of thesystem. The GOS of a data communications system is typically defined asthe total delay incurred in the transfer of a particular amount of data.

Yet another significant difference between voice and data services isthat the former require a reliable communications link that, in a CDMAsystem, is provided by soft handoff. Soft handoff results in redundanttransmissions from two or more base stations to improve reliability.However, this additional reliability may not be required for datatransmission because data frames received in error may be retransmitted.For data services, the transmit power needed to support soft handoff maybe more efficiently used for transmitting additional data.

Because of the significant differences noted above, it is a challenge todesign a communications system capable of efficiently supporting bothvoice and data services. The IS-95 CDMA system is designed toefficiently transmit voice data, and is also capable of transmittingtraffic data. The designs of the channel structure and the data frameformat pursuant to IS-95 have been optimized for voice data. Acommunications system based on IS-95 that is enhanced for data servicesis disclosed in U.S. patent application Ser. No. 08/963,386, entitled“METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filedNov. 3, 1997, now U.S. Pat. No. 6,574,211 issued on Jun. 3, 2003,assigned to the assignee of the present invention and incorporatedherein by reference.

Given the ever-growing demand for wireless voice and data communication,however a higher efficiency, higher performance wireless communicationssystem capable of supporting voice and data services is desirable.

SUMMARY OF THE INVENTION

The present invention provides a novel and improved communicationssystem capable of supporting multiple types of services having differentdelay requirements. Such types of services may include, for example,“full duplex real time” (FDRT) services that require short one-way delay(e.g., voice), “half duplex real time” (HDRT) services that can toleratelonger one-way delay, as long as the delay does not vary by a largeamount (e.g., video, audio), “non-real time” (NRT) services that are notquite as sensitive to delays (e.g., packet data), and others. Data forthese different types of services can be efficiently transmitted usingvarious mechanisms, some of which are described below.

An embodiment of the invention provides a transmitter unit for use in amulti-carrier (e.g., OFDM) communication system and configurable tosupport multiple types of services. The transmitter unit includes one ormore encoders, a symbol mapping element, and a modulator. Each encoderreceives and codes a respective channel data stream to generate acorresponding coded data stream. The symbol mapping element receives andmaps data from the coded data streams to generate modulation symbolvectors, with each modulation symbol vector including a set of datavalues used to modulate a set of tones to generate an OFDM symbol. Thedata from each coded data stream is mapped to a respective set of one ormore “circuits”, with each circuit including a particular set of one ormore tones. The modulator modulates the modulation symbol vectors toprovide a modulated signal suitable for transmission. The transmitterunit can further include a set of scaling elements that scale the codeddata streams with a set of scaling factors to provide power adjustment.

The modulator can include an inverse Fourier transform, a cyclic prefixgenerator, and an upconverter. The inverse Fourier transform receivesthe modulation symbol vectors and generates a time-domain representationof each modulation symbol vector to provide the corresponding OFDMsymbol. The cyclic prefix generator repeats a portion of each OFDMsymbol to generate a corresponding transmission symbol, and theupconverter modulates the transmission symbols to generate the modulatedsignal.

Each circuit can be defined to include a number of tones from a numberof OFDM symbols (to provide temporal and frequency diversity), a numberof tones from a single OFDM symbol, all tones from one or more OFDMsymbols, or some other combination of tones. The circuits can have equalsize or different sizes.

The data for each channel data stream can be transmitted in packets.Each packet can be defined to include various fields, depending on theparticular implementation. In one implementation, each packet includespacket type identifier indicative of a change in the circuit to be usedto transmit the next packet, a circuit identifier indicative of aparticular circuit to be used to transmit the next packet, and a datafield for the payload. In another implementation, each packet includes auser identifier indicative of an intended recipient of the packet and adata field for the payload.

The channel data streams can be transmitted over slots, with each slotincluding a number of OFDM symbols. Each slot can be further dividedinto two or more partitions, with each partition including one or moreOFDM symbols and used to support one or more types of service. Forexample, one partition of each slot can be used to support full duplexreal time services having a short delay requirements and anotherpartition of each slot can be used to support half duplex real timeand/or non-real time services having more relaxed delay requirements.

For improved efficiency, full rate data for a particular channel datastream can be transmitted via a first circuit and lower rate data can betransmitted via a second circuit. The second circuit can be transmittedevery X number of slots (X>1) or can be a lower capacity circuit. Anindication to use a new circuit can be sent in a field of the packettransmitted on the current circuit or can be sent via a control channel.The new circuit may be utilized after receiving an acknowledgment of thereceipt of such indication to use the new circuit.

In another specific implementation, the transmitter unit includes one ormore cover elements coupled to the respective encoders. Each coverelement receives and covers a respective coded data stream with aparticular Walsh sequence assigned to that coded data stream to generatea corresponding covered data stream. The scaling elements then scale thecovered data streams with respective scaling factors to generate scaleddata streams. A summer receives and sums the scaled data streams toprovide a combined data stream that is then provided to the modulator.Each Walsh sequence can be transmitted over multiple tones of each ofthe OFDM symbols used for the Walsh sequence. Also, the length of theWalsh sequence can be matched to the number of tones in each OFDMsymbol. For example, Walsh sequences of length 128 can be used for OFDMsymbols having 128 tones, and the 128 chips of each Walsh sequence canbe transmitted on the 128 tones of one OFDM symbol.

Another embodiment of the invention provides a method for generating andtransmitting a modulated signal capable of supporting multiple types ofservices. In accordance with the method, one or more channel datastreams are received, and each channel data stream is coded with aparticular coding scheme to generate a corresponding coded data stream.The data from the coded data streams are mapped to generate modulationsymbol vectors, with each modulation symbol vector including a number ofdata values used to modulate a number of tones to generate an OFDMsymbol. The data from each coded data stream is mapped to a respectiveset of one or more circuits, with each circuit including a respectiveset of one or more tones. The coded data streams can be scaled withrespective scaling factors to provide power adjustment. The modulationsymbol vectors are then modulated to provide a modulated signal suitablefor transmission.

To perform multi-carrier modulation, each modulation symbol vector isfirst transformed to a time-domain representation to provide acorresponding OFDM symbol. A portion of each OFDM symbol is thenrepeated to generate a corresponding transmission symbol, and thetransmission symbols are further processed to generate the modulatedsignal.

The invention further provides a receiver unit capable of receiving andprocessing the modulated signal generated in the manner described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a diagram of a multiple-input multiple-output (MIMO)communications system;

FIG. 2 is a diagram that graphically illustrates a specific example of atransmission from a transmit antenna at a transmitter unit;

FIG. 3 is a block diagram of an embodiment of a data processor and amodulator of the communications system shown in FIG. 1;

FIGS. 4A and 4B are block diagrams of two embodiments of a channel dataprocessor that can be used for processing one channel data steam such ascontrol, broadcast, voice, or traffic data;

FIGS. 5A through 5C are block diagrams of an embodiment of theprocessing units that can be used to generate the transmit signal shownin FIG. 2;

FIG. 6 is a block diagram of an embodiment of a receiver unit, havingmultiple receive antennas, which can be used to receive one or morechannel data streams;

FIG. 7 shows plots that illustrate the spectral efficiency achievablewith some of the operating modes of a communications system inaccordance with one embodiment;

FIG. 8A is a diagram of an embodiment of a structure that can be used totransmit various types of services;

FIGS. 8B and 8C are diagrams of a specific embodiment of two packetstructures that can be used for transmitting data;

FIG. 9 is a block diagram of an embodiment of a data processor and amodulator that can be used to multiplex multiple users on orthogonalOFDM tones;

FIG. 10 is a block diagram of an embodiment of a data processor and amodulator that can be used to multiplex multiple users on the same OFDMtones using orthogonal (e.g., Walsh) codes;

FIG. 11 is a diagram of a OFDM-based MIMO system wit feedback of channelstate information; and

FIG. 12 is a diagram of an exploratory OFDM pilot signal structure thatcan be used to estimate the channel state information.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 1 is a diagram of a multiple-input multiple-output (MIMO)communications system 100 capable of implementing some embodiments ofthe invention. Communications system 100 can be operative to provide acombination of antenna, frequency, and temporal diversity to increasespectral efficiency, improve performance, and enhance flexibility.Increased spectral efficiency is characterized by the ability totransmit more bits per second per Hertz (bps/Hz) when and where possibleto better utilize the available system bandwidth. Techniques to obtainhigher spectral efficiency are described in further detail below.Improved performance may be quantified, for example, by a lowerbit-error-rate (BER) or frame-error-rate (FER) for a given linkcarrier-to-noise-plus-interference ratio (C/I). And enhanced flexibilityis characterized by the ability to accommodate multiple users havingdifferent and typically disparate requirements. These goals may beachieved, in part, by employing multi-carrier modulation, time divisionmultiplexing (TDM), multiple transmit and/or receive antennas, and othertechniques. The features, aspects, and advantages of the invention aredescribed in further detail below.

As shown in FIG. 1, communications system 100 includes a first system110 in communication with a second system 120. System 110 includes a(transmit) data processor 112 that (1) receives or generates data, (2)processes the data to provide antenna, frequency, or temporal diversity,or a combination thereof, and (3) provides processed modulation symbolsto a number of modulators (MOD) 114 a through 114 t. Each modulator 114further processes the modulation symbols and generates an RF modulatedsignal suitable for transmission. The RF modulated signals frommodulators 114 a through 114 t are then transmitted from respectiveantennas 116 a through 116 t over communications links 118 to system120.

In the embodiment shown in FIG. 1, system 120 includes a number ofreceive antennas 122 a through 122 r that receive the transmittedsignals and provide the received signals to respective demodulators(DEMOD) 124 a through 124 r. As shown in FIG. 1, each receive antenna122 may receive signals from one or more transmit antennas 116 dependingon a number of factors such as, for example, the operating mode used atsystem 110, the directivity of the transmit and receive antennas, thecharacteristics of the communications links, and others. Eachdemodulator 124 demodulates the respective received signal using ademodulation scheme that is complementary to the modulation scheme usedat the transmitter. The demodulated symbols from demodulators 124 athrough 124 r are then provided to a (receive) data processor 126 thatfurther processes the symbols to provide the output data. The dataprocessing at the transmitter and receiver units is described in furtherdetail below.

FIG. 1 shows only the forward link transmission from system 110 tosystem 120. This configuration may be used for data broadcast and otherone-way data transmission applications. In a bi-directionalcommunications system, a reverse link from system 120 to system 110 isalso provided, although not shown in FIG. 1 for simplicity. For thebi-directional communications system, each of systems 110 and 120 mayoperate as a transmitter unit or a receiver unit, or both concurrently,depending on whether data is being transmitted from, or received at, theunit.

For simplicity, communications system 100 is shown to include onetransmitter unit (i.e., system 110) and one receiver unit (i.e., system120). However, other variations and configurations of the communicationssystem are possible. For example, in a multi-user, multiple accesscommunications system, a single transmitter unit may be used toconcurrently transmit data to a number of receiver units. Also, in amanner similar to soft-handoff in an IS-95 CDMA system, a receiver unitmay concurrently receive transmissions from a number of transmitterunits. The communications system of the invention may include any numberof transmitter and receiver units.

Each transmitter unit may include a single transmit antenna or a numberof transmit antennas, such as that shown in FIG. 1. Similarly, eachreceiver unit may include a single receive antenna or a number ofreceive antennas, again such as that shown in FIG. 1. For example, thecommunications system may include a central system (i.e., similar to abase station in the IS-95 CDMA system) having a number of antennas thattransmit data to, and receive data from, a number of remote systems(i.e., subscriber units, similar to remote stations in the CDMA system),some of which may include one antenna and others of which may includemultiple antennas. Generally, as the number of transmit and receiveantennas increases, antenna diversity increases and performanceimproves, as described below.

As used herein, an antenna refers to a collection of one or more antennaelements that are distributed in space. The antenna elements may bephysically located at a single site or distributed over multiple sites.Antenna elements physically co-located at a single site may be operatedas an antenna array (e.g., such as for a CDMA base station). An antennanetwork consists of a collection of antenna arrays or elements that arephysically separated (e.g., several CDMA base stations). An antennaarray or an antenna network may be designed with the ability to formbeams and to transmit multiple beams from the antenna array or network.For example, a CDMA base station may be designed with the capability totransmit up to three beams to three different sections of a coveragearea (or sectors) from the same antenna array. Thus, the three beams maybe viewed as three transmissions from three antennas.

The communications system of the invention can be designed to provide amulti-user, multiple access communications scheme capable of supportingsubscriber units having different requirements as well as capabilities.The scheme allows the system's total operating bandwidth, W, (e.g.,1.2288 MHz) to be efficiently shared among different types of servicesthat may have highly disparate data rate, delay, and quality of service(QOS) requirements.

Examples of such disparate types of services include voice services anddata services. Voice services are typically characterized by a low datarate (e.g., 8 kbps to 32 kbps), short processing delay (e.g., 3 msec to100 msec overall one-way delay), and sustained use of a communicationschannel for an extended period of time. The short delay requirementsimposed by voice services typically require a small fraction of thesystem resources to be dedicated to each voice call for the duration ofthe call. In contrast, data services are characterized by “bursty”traffics in which variable amounts of data are sent at sporadic times.The amount of data can vary significantly from burst-to-burst and fromuser-to-user. For high efficiency, the communications system of theinvention can be designed with the capability to allocate a portion ofthe available resources to voice services as required and the remainingresources to data services. In some embodiments of the invention, afraction of the available system resources may also be dedicated forcertain data services or certain types of data services.

The distribution of data rates achievable by each subscriber unit canvary widely between some minimum and maximum instantaneous values (e.g.,from 200 kbps to over 20 Mbps). The achievable data rate for aparticular subscriber unit at any given moment may be influenced by anumber of factors such as the amount of available transmit power, thequality of the communications link (i.e., the C/I), the coding scheme,and others. The data rate requirement of each subscriber unit may alsovary widely between a minimum value (e.g., 8 kbps, for a voice call) allthe way up to the maximum supported instantaneous peak rate (e.g., 20Mbps for bursty data services).

The percentage of voice and data traffic is typically a random variablethat changes over time. In accordance with certain aspects of theinvention, to efficiently support both types of services concurrently,the communications system of the invention is designed with thecapability to dynamic allocate the available resources based on theamount of voice and data traffic. A scheme to dynamically allocateresources is described below. Another scheme to allocate resources isdescribed in the aforementioned U.S. patent application Ser. No.08/963,386, now U.S. Pat. No. 6,574,211 issued on Jun. 3, 2003.

The communications system of the invention provides the above-describedfeatures and advantages, and is capable of supporting different types ofservices having disparate requirements. The features are achieved byemploying antenna, frequency, or temporal diversity, or a combinationthereof. In some embodiments of the invention, antenna, frequency, ortemporal diversity can be independently achieved and dynamicallyselected.

As used herein, antenna diversity refers to the transmission and/orreception of data over more than one antenna, frequency diversity refersto the transmission of data over more than one sub-band, and temporaldiversity refers to the transmission of data over more than one timeperiod. Antenna, frequency, and temporal diversity may includesubcategories. For example, transmit diversity refers to the use of morethan one transmit antenna in a manner to improve the reliability of thecommunications link, receive diversity refers to the use of more thanone receive antenna in a manner to improve the reliability of thecommunications link, and spatial diversity refers to the use of multipletransmit and receive antennas to improve the reliability and/or increasethe capacity of the communications link. Transmit and receive diversitycan also be used in combination to improve the reliability of thecommunications link without increasing the link capacity. Variouscombinations of antenna, frequency, and temporal diversity can thus beachieved and are within the scope of the present invention.

Frequency diversity can be provided by use of a multi-carrier modulationscheme such as orthogonal frequency division multiplexing (OFDM), whichallows for transmission of data over various sub-bands of the operatingbandwidth. Temporal diversity is achieved by transmitting the data overdifferent times, which can be more easily accomplished with the use oftime-division multiplexing (TDM). These various aspects of thecommunications system of the invention are described in further detailbelow.

In accordance with an aspect of the invention, antenna diversity isachieved by employing a number of (N_(T)) transmit antennas at thetransmitter unit or a number of (N_(R)) receive antennas at the receiverunit, or multiple antennas at both the transmitter and receiver units.In a terrestrial communications system (e.g., a cellular system, abroadcast system, an MMDS system, and others), an RF modulated signalfrom a transmitter unit may reach the receiver unit via a number oftransmission paths. The characteristics of the transmission pathstypically vary over time based on a number of factors. If more than onetransmit or receive antenna is used, and if the transmission pathsbetween the transmit and receive antennas are independent (i.e.,uncorrelated), which is generally true to at least an extent, then thelikelihood of correctly receiving the transmitted signal increases asthe number of antennas increases. Generally, as the number of transmitand receive antennas increases, diversity increases and performanceimproves.

In some embodiments of the invention, antenna diversity is dynamicallyprovided based on the characteristics of the communications link toprovide the required performance. For example, higher degree of antennadiversity can be provided for some types of communication (e.g.,signaling), for some types of services (e.g., voice), for somecommunications link characteristics (e.g., low C/I), or for some otherconditions or considerations.

As used herein, antenna diversity includes transmit diversity andreceive diversity. For transmit diversity, data is transmitted overmultiple transmit antennas. Typically, additional processing isperformed on the data transmitted from the transmit antennas to achievedthe desired diversity. For example, the data transmitted from differenttransmit antennas may be delayed or reordered in time, or coded andinterleaved across the available transmit antennas. Also, frequency andtemporal diversity may be used in conjunction with the differenttransmit antennas. For receive diversity, modulated signals are receivedon multiple receive antennas, and diversity is achieved by simplyreceiving the signals via different transmission paths.

In accordance with another aspect of the invention, frequency diversitycan be achieved by employing a multi-carrier modulation scheme. One suchscheme that has numerous advantages is OFDM. With OFDM modulation, theoverall transmission channel is essentially divided into a number of (L)parallel sub-channels that are used to transmit the same or differentdata. The overall transmission channel occupies the total operatingbandwidth of W, and each of the sub-channels occupies a sub-band havinga bandwidth of W/L and centered at a different center frequency. Eachsub-channel has a bandwidth that is a portion of the total operatingbandwidth. Each of the sub-channels may also be considered anindependent data transmission channel that may be associated with aparticular (and possibly unique) processing, coding, and modulationscheme, as described below.

The data may be partitioned and transmitted over any defined set of twoor more sub-bands to provide frequency diversity. For example, thetransmission to a particular subscriber unit may occur over sub-channel1 at time slot 1, sub-channel 5 at time slot 2, sub-channel 2 at timeslot 3, and so on. As another example, data for a particular subscriberunit may be transmitted over sub-channels 1 and 2 at time slot 1 (e.g.,with the same data being transmitted on both sub-channels), sub-channels4 and 6 at time slot 2, only sub-channel 2 at time slot 3, and so on.Transmission of data over different sub-channels over time can improvethe performance of a communications system experiencing frequencyselective fading and channel distortion. Other benefits of OFDMmodulation are described below.

In accordance with yet another aspect of the invention, temporaldiversity is achieved by transmitting data at different times, which canbe more easily accomplished using time division multiplexing (TDM). Fordata services (and possibly for voice services), data transmissionoccurs over time slots that may be selected to provide immunity to timedependent degradation in the communications link. Temporal diversity mayalso be achieved through the use of interleaving.

For example, the transmission to a particular subscriber unit may occurover time slots 1 through x, or on a subset of the possible time slotsfrom 1 through x (e.g., time slots 1, 5, 8, and so on). The amount ofdata transmitted at each time slot may be variable or fixed.Transmission over multiple time slots improves the likelihood of correctdata reception due to, for example, impulse noise and interference.

The combination of antenna, frequency, and temporal diversity allows thecommunications system of the invention to provide robust performance.Antenna, frequency, and/or temporal diversity improves the likelihood ofcorrect reception of at least some of the transmitted data, which maythen be used (e.g., through decoding) to correct for some errors thatmay have occurred in the other transmissions. The combination ofantenna, frequency, and temporal diversity also allows thecommunications system to concurrently accommodate different types ofservices having disparate data rate, processing delay, and quality ofservice requirements.

The communications system of the invention can be designed and operatedin a number of different communications modes, with each communicationsmode employing antenna, frequency, or temporal diversity, or acombination thereof. The communications modes include, for example, adiversity communications mode and a MIMO communications mode. Variouscombinations of the diversity and MIMO communications modes can also besupported by the communications system. Also, other communications modescan be implemented and are within the scope of the present invention.

The diversity communications mode employs transmit and/or receivediversity, frequency, or temporal diversity, or a combination thereof,and is generally used to improve the reliability of the communicationslink. In one implementation of the diversity communications mode, thetransmitter unit selects a modulation and coding scheme (i.e.,configuration) from a finite set of possible configurations, which areknown to the receiver units. For example, each overhead and commonchannel may be associated with a particular configuration that is knownto all receiver units. When using the diversity communications mode fora specific user (e.g., for a voice call or a data transmission), themode and/or configuration may be known a priori (e.g., from a previousset up) or negotiated (e.g., via a common channel) by the receiver unit.

In the diversity communications mode, data is transmitted on one or moresub-channels, from one or more antennas, and at one or more timeperiods. The allocated sub-channels may be associated with the sameantenna, or may be sub-channels associated with different antennas. In acommon application of the diversity communications mode, which is alsoreferred to as a “pure” diversity communications mode, data istransmitted from all available transmit antennas to the destinationreceiver unit. The pure diversity communications mode can be used ininstances where the data rate requirements are low or when the C/I islow, or when both are true.

The MIMO communications mode employs antenna diversity at both ends ofthe communication link and is generally used to improve both thereliability and increase the capacity of the communications link. TheMIMO communications mode may further employ frequency and/or temporaldiversity in combination with the antenna diversity. The MIMOcommunications mode, which may also be referred to herein as the spatialcommunications mode, employs one or more processing modes to bedescribed below.

The diversity communications mode generally has lower spectralefficiency than the MIMO communications mode, especially at high C/Ilevels. However, at low to moderate C/I values, the diversitycommunications mode achieves comparable efficiency and can be simpler toimplement. In general, the use of the MIMO communications mode providesgreater spectral efficiency when used, particularly at moderate to highC/I values. The MIMO communications mode may thus be advantageously usedwhen the data rate requirements are moderate to high.

The communications system can be designed to concurrently support bothdiversity and MIMO communications modes. The communications modes can beapplied in various manners and, for increased flexibility, may beapplied independently on a sub-channel basis. The MIMO communicationsmode is typically applied to specific users. However, eachcommunications mode may be applied on each sub-channel independently,across a subset of sub-channels, across all sub-channels, or on someother basis. For example, the use of the MIMO communications mode may beapplied to a specific user (e.g., a data user) and, concurrently, theuse of the diversity communications mode may be applied to anotherspecific user (e.g., a voice user) on a different sub-channel. Thediversity communications mode may also be applied, for example, onsub-channels experiencing higher path loss.

The communications system of the invention can also be designed tosupport a number of processing modes. When the transmitter unit isprovided with information indicative of the conditions (i.e., the“state”) of the communications links, additional processing can beperformed at the transmitter unit to further improve performance andincrease efficiency. Full channel state information (CSI) or partial CSImay be available to the transmitter unit. Full CSI includes sufficientcharacterization of the propagation path (i.e., amplitude and phase)between all pairs of transmit and receive antennas for each sub-band.Full CSI also includes the C/I per sub-band. The full CSI may beembodied in a set of matrices of complex gain values that aredescriptive of the conditions of the transmission paths from thetransmit antennas to the receive antennas, as described below. PartialCSI may include, for example, the C/I of the sub-band. With full CSI orpartial CSI, the transmitter unit pre-conditions the data prior totransmission to receiver unit.

In a specific implementation of the full-CSI processing mode, thetransmitter unit preconditions the signals presented to the transmitantennas in a way that is unique to a specific receiver unit (e.g., thepre-conditioning is performed for each sub-band assigned to thatreceiver unit). As long as the channel does not change appreciably fromthe time it is measured by the receiver unit and subsequently sent backto the transmitter and used to precondition the transmission, theintended receiver unit can demodulate the transmission. In thisimplementation, a full-CSI based MIMO communication can only bedemodulated by the receiver unit associated with the CSI used toprecondition the transmitted signals.

In a specific implementation of the partial-CSI or no-CSI processingmodes, the transmitter unit employs a common modulation and codingscheme (e.g., on each data channel transmission), which then can be (intheory) demodulated by all receiver units. In an implementation of thepartial-CSI processing mode, a single receiver unit can specify its C/I,and the modulation employed on all antennas, can be selected accordingly(e.g., for reliable transmission) for that receiver unit. Other receiverunits can attempt to demodulate the transmission and, if they haveadequate C/I, may be able to successfully recover the transmission. Acommon (e.g., broadcast) channel can use a no-CSI processing mode toreach all users.

The full-CSI processing is briefly described below. When the CSI isavailable at the transmitter unit, a simple approach is to decompose themulti-input multi-output channel into a set of independent channels.Given the channel transfer function at the transmitters, the lefteigenvectors may be used to transmit different data streams. Themodulation alphabet used with each eigenvector is determined by theavailable C/I of that mode, given by the eigenvalues. If H is the N_(R)xN_(T) matrix that gives the channel response for the N_(T) transmitterantenna elements and N_(R) receiver antenna elements at a specific time,and x is the N_(T)-vector of inputs to the channel, then the receivedsignal can be expressed as:y=Hx+n,where n is an N_(R)-vector representing noise plus interference. Theeigenvector decomposition of the Hermitian matrix formed by the productof the channel matrix with its conjugate-transpose can be expressed as:H*H=EΛE*,where the symbol * denotes conjugate-transpose, E is the eigenvectormatrix, and Λ is a diagonal matrix of eigenvalues, both of dimensionN_(T)xN_(T). The transmitter converts a set of N_(T) modulation symbolsb using the eigenvector matrix E. The transmitted modulation symbolsfrom the N_(T) transmit antennas can thus be expressed as:x=Eb.For all antennas, the pre-conditioning can thus be achieved by a matrixmultiply operation expressed as:

$\begin{matrix}{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{N_{T}}\end{bmatrix} = {\begin{bmatrix}{e_{11},} & {e_{12},} & e_{1N_{\;_{T}}} \\{e_{21},} & {e_{22},} & e_{2N_{T}} \\{e_{N_{T}1},} & {e_{N_{T}1},} & e_{N_{T}N_{T}}\end{bmatrix} \cdot \begin{bmatrix}b_{1} \\b_{2} \\\vdots \\b_{N_{T}}\end{bmatrix}}} & {{Eq}\mspace{14mu}(1)}\end{matrix}$where b₁, b₂, . . . and b_(NT) are respectively the modulation symbolsfor a particular sub-channel at transmit antennas 1, 2, . . . N_(T),where each modulation symbol can be generated using, for example, M-PSK,M-QAM, and so on, as described below;

-   -   E=is the eigenvector matrix related to the transmission loss        from transmit antennas to the receive antennas; and    -   x₁, x₂, . . . x_(NT) are the pre-conditioned modulation symbols,        which can be expressed as:        x ₁ =b ₁ ·e ₁₁ +b ₂ ·e ₁₂ + . . . +b _(N) _(T) ·e _(1N) _(T) ,        x ₂ =b ₁ ·e ₂₁ +b ₂ ·e ₂₂ + . . . +b _(N) _(T) ·e _(2N) _(T) ,        and        x _(N) _(T) =b ₁ ·e _(N) _(T) ₁ +b ₂ ·e _(N) _(T) ₂ + . . . +b        _(N) _(T) ·e _(N) _(T) _(N) _(T) .        Since H*H is Hermitian, the eigenvector matrix is unitary. Thus,        if the elements of b have equal power, the elements of x also        have equal power. The received signal may then be expressed as:        y=HEb+n.

The receiver performs a channel-matched-filter operation, followed bymultiplication by the right eigenvectors. The result of thechannel-matched-filter operation is the vector z, which can be expressedas:z=E*H*HEb+E*H*n=Λb+{circumflex over (n)},  Eq.(2)where the new noise term has covariance that can be expressed as:E( {circumflex over (n)}{circumflex over (n)} )=E(E*H*nnHE)=E*H*HE=Λ,i.e., the noise components are independent with variance given by theeigenvalues. The C/I of the i-th component of z is λ_(i), the i-thdiagonal element of Λ.

The transmitter unit can thus select a modulation alphabet (i.e., signalconstellation) for each of the eigenvectors based on the C/I that isgiven by the eigenvalue. Providing that the channel conditions do notchange appreciably in the interval between the time the CSI is measuredat the receiver and reported and used to precondition the transmissionat the transmitter, the performance of the communications system willthen be equivalent to that of a set of independent AWGN channels withknown C/I's.

As an example, assume that the MIMO communications mode is applied to achannel data stream that is transmitted on one particular sub-channelfrom four transmit antennas. The channel data stream is demultiplexedinto four data sub-streams, one data sub-stream for each transmitantenna. Each data sub-stream is then modulated using a particularmodulation scheme (e.g., M-PSK, M-QAM, or other) selected based on theCSI for that sub-band and for that transmit antenna. Four modulationsub-streams are thus generated for the four data sub-streams, with eachmodulation sub-streams including a stream of modulation symbols. Thefour modulation sub-streams are then pre-conditioned using theeigenvector matrix, as expressed above in equation (1), to generatepre-conditioned modulation symbols. The four streams of pre-conditionedmodulation symbols are respectively provided to the four combiners ofthe four transmit antennas. Each combiner combines the receivedpre-conditioned modulation symbols with the modulation symbols for theother sub-channels to generate a modulation symbol vector stream for theassociated transmit antenna.

The full-CSI based processing is typically employed in the MIMOcommunications mode where parallel data streams are transmitted to aspecific user on each of the channel eigenmodes for the each of theallocated sub-channels. Similar processing based on full CSI can beperformed where transmission on only a subset of the availableeigenmodes is accommodated in each of the allocated sub-channels (e.g.,to implement beam steering). Because of the cost associated with thefull-CSI processing (e.g., increased complexity at the transmitter andreceiver units, increased overhead for the transmission of the CSI fromthe receiver unit to the transmitter unit, and so on), full-CSIprocessing can be applied in certain instances in the MIMOcommunications mode where the additional increase in performance andefficiency is justified.

In instances where full CSI is not available, less descriptiveinformation on the transmission path (or partial CSI) may be availableand can be used to pre-condition the data prior to transmission. Forexample, the C/I of each of the sub-channels may be available. The C/Iinformation can then be used to control the transmission from varioustransmit antennas to provide the required performance in thesub-channels of interest and increase system capacity.

As used herein, full-CSI based processing modes denote processing modesthat use full CSI, and partial-CSI based processing modes denoteprocessing modes that use partial CSI. The full-CSI based processingmodes include, for example, the full-CSI MIMO mode that utilizesfull-CSI based processing in the MIMO communications mode. Thepartial-CSI based modes include, for example, the partial-CSI MIMO modethat utilizes partial-CSI based processing in the MIMO communicationsmode.

In instances where full-CSI or partial-CSI processing is employed toallow the transmitter unit to pre-condition the data using the availablechannel state information (e.g., the eigenmodes or C/I), feedbackinformation from the receiver unit is required, which uses a portion ofthe reverse link capacity. Therefore, there is a cost associated withthe full-CSI and the partial-CSI based processing modes. The cost shouldto be factored into the choice of which processing mode to employ. Thepartial-CSI based processing mode requires less overhead and may be moreefficient in some instances. The no-CSI based processing mode requiresno overhead and may also be more efficient than the full-CSI basedprocessing mode or the partial-CSI based processing mode under someother circumstances.

If the transmitter unit has CSI and uses the eigenmodes representativeof the characteristics of the communications links to transmitindependent channel data streams, then the sub-channels allocated inthis case are typically uniquely assigned to a single user. On the otherhand, if the modulation and coding scheme employed is common for allusers (i.e., the CSI employed at the transmitter is not user-specific),then it is possible that information transmitted in this processing modecould be received and decoded by more than one user, depending on theirC/I.

FIG. 2 is a diagram that graphically illustrates at least some of theaspects of the communications system of the invention. FIG. 2 shows aspecific example of a transmission from one of N_(T) transmit antennasat a transmitter unit. In FIG. 2, the horizontal axis is time and thevertical axis is frequency. In this example, the transmission channelincludes 16 sub-channels and is used to transmit a sequence of OFDMsymbols, with each OFDM symbol covering all 16 sub-channels (one OFDMsymbol is indicated at the top of FIG. 2 and includes all 16 sub-bands).A TDM structure is also illustrated in which the data transmission ispartitioned into time slots, with each time slot having the duration of,for example, the length of one modulation symbol (i.e., each modulationsymbol is used as the TDM interval).

The available sub-channels can be used to transmit signaling, voice,traffic data, and others. In the example shown in FIG. 2, the modulationsymbol at time slot 1 corresponds to pilot data, which is periodicallytransmitted to assist the receiver units synchronize and perform channelestimation. Other techniques for distributing pilot data over time andfrequency can also be used and are within the scope of the presentinvention. In addition, it may be advantageous to utilize a particularmodulation scheme during the pilot interval if all sub-channels areemployed (e.g., a PN code with a chip duration of approximately 1/W).Transmission of the pilot modulation symbol typically occurs at aparticular frame rate, which is usually selected to be fast enough topermit accurate tracking of variations in the communications link.

The time slots not used for pilot transmissions can then be used totransmit various types of data. For example, sub-channels 1 and 2 may bereserved for the transmission of control and broadcast data to thereceiver units. The data on these sub-channels is generally intended tobe received by all receiver units. However, some of the messages on thecontrol channel may be user specific, and can be encoded accordingly.

Voice data and traffic data can be transmitted in the remainingsub-channels. For the example shown in FIG. 2, sub-channel 3 at timeslots 2 through 9 is used for voice call 1, sub-channel 4 at time slots2 through 9 is used for voice call 2, sub-channel 5 at time slots 5through 9 is used for voice call 3, and sub-channel 6 at time slots 7through 9 is used for voice call 5.

The remaining available sub-channels and time slots may be used fortransmissions of traffic data. In the example shown in FIG. 2, data 1transmission uses sub-channels 5 through 16 at time slot 2 andsub-channels 7 through 16 at time slot 7, data 2 transmission usessub-channels 5 through 16 at time slots 3 and 4 and sub-channels 6through 16 at time slots 5, data 3 transmission uses sub-channels 6through 16 at time slot 6, data 4 transmission uses sub-channels 7through 16 at time slot 8, data 5 transmission uses sub-channels 7through 11 at time slot 9, and data 6 transmission uses sub-channels 12through 16 at time slot 9. Data 1 through 6 transmissions can representtransmissions of traffic data to one or more receiver units.

The communications system of the invention flexibly supports thetransmissions of traffic data. As shown in FIG. 2, a particular datatransmission (e.g., data 2) may occur over multiple sub-channels and/ormultiple time slots, and multiple data transmissions (e.g., data 5 and6) may occur at one time slot. A data transmission (e.g., data 1) mayalso occur over non-contiguous time slots. The system can also bedesigned to support multiple data transmissions on one sub-channel. Forexample, voice data may be multiplexed with traffic data and transmittedon a single sub-channel.

The multiplexing of the data transmissions can potentially change fromOFDM symbol to symbol. Moreover, the communications mode may bedifferent from user to user (e.g., from one voice or data transmissionto other). For example, the voice users may use the diversitycommunications mode, and the data users may use the MIMO communicationsmodes. These features concept can be extended to the sub-channel level.For example, a data user may use the MIMO communications mode insub-channels that have sufficient C/I and the diversity communicationsmode in remaining sub-channels.

Antenna, frequency, and temporal diversity may be respectively achievedby transmitting data from multiple antennas, on multiple sub-channels indifferent sub-bands, and over multiple time slots. For example, antennadiversity for a particular transmission (e.g., voice call 1) may beachieved by transmitting the (voice) data on a particular sub-channel(e.g., sub-channel 1) over two or more antennas. Frequency diversity fora particular transmission (e.g., voice call 1) may be achieved bytransmitting the data on two or more sub-channels in different sub-bands(e.g., sub-channels 1 and 2). A combination of antenna and frequencydiversity may be obtained by transmitting data from two or more antennasand on two or more sub-channels. Temporal diversity may be achieved bytransmitting data over multiple time slots. For example, as shown inFIG. 2, data 1 transmission at time slot 7 is a portion (e.g., new orrepeated) of the data 1 transmission at time slot 2.

The same or different data may be transmitted from multiple antennasand/or on multiple sub-bands to obtain the desired diversity. Forexample, the data may be transmitted on: (1) one sub-channel from oneantenna, (2) one sub-channel (e.g., sub-channel 1) from multipleantennas, (3) one sub-channel from all N_(T) antennas, (4) a set ofsub-channels (e.g., sub-channels 1 and 2) from one antenna, (5), a setof sub-channels from multiple antennas, (6) a set of sub-channels fromall N_(T) antennas, or (7) a set of channels from a set of antennas(e.g., sub-channel 1 from antennas 1 and 2 at one time slot,sub-channels 1 and 2 from antenna 2 at another time slot, and so on).Thus, any combination of sub-channels and antennas may be used toprovide antenna and frequency diversity.

In accordance with certain embodiments of the invention that provide themost flexibility and are capable of achieving high performance andefficiency, each sub-channel at each time slot for each transmit antennamay be viewed as an independent unit of transmission (i.e., a modulationsymbol) that can be used to transmit any type of data such as pilot,signaling, broadcast, voice, traffic data, and others, or a combinationthereof (e.g., multiplexed voice and traffic data). In such design, avoice call may be dynamically assigned different sub-channels over time.

Flexibility, performance, and efficiency are further achieved byallowing for independence among the modulation symbols, as describedbelow. For example, each modulation symbol may be generated from amodulation scheme (e.g., M-PSK, M-QAM, and others) that results in thebest use of the resource at that particular time, frequency, and space.

A number of constraints may be placed to simplify the design andimplementation of the transmitter and receiver units. For example, avoice call may be assigned to a particular sub-channel for the durationof the call, or until such time as a sub-channel reassignment isperformed. Also, signaling and/or broadcast data may be designated tosome fixed sub-channels (e.g., sub-channel 1 for control data andsub-channel 2 for broadcast data, as shown FIG. 2) so that the receiverunits know a priori which sub-channels to demodulate to receive thedata.

Also, each data transmission channel or sub-channel may be restricted toa particular modulation scheme (e.g., M-PSK, M-QAM) for the duration ofthe transmission or until such time as a new modulation scheme isassigned. For example, in FIG. 2, voice call 1 on sub-channel 3 may useQPSK, voice call 2 on sub-channel 4 may use 16-QAM, data 1 transmissionat time slot 2 may use 8-PSK, data 2 transmission at time slots 3through 5 may use 16-QAM, and so on.

The use of TDM allows for greater flexibility in the transmission ofvoice data and traffic data, and various assignments of resources can becontemplated. For example, a user can be assigned one sub-channel foreach time slot or, equivalently, four sub-channels every fourth timeslot, or some other allocations. TDM allows for data to be aggregatedand transmitted at designated time slot(s) for improved efficiency.

If voice activity is implemented at the transmitter, then in theintervals where no voice is being transmitted, the transmitter mayassign other users to the sub-channel so that the sub-channel efficiencyis maximized. In the event that no data is available to transmit duringthe idle voice periods, the transmitter can decrease (or turn-off) thepower transmitted in the sub-channel, reducing the interference levelspresented to other users in the system that are using the samesub-channel in another cell in the network. The same feature can be alsoextended to the overhead, control, data, and other channels.

Allocation of a small portion of the available resources over acontinuous time period typically results in lower delays, and may bebetter suited for delay sensitive services such as voice. Transmissionusing TDM can provide higher efficiency, at the cost of possibleadditional delays. The communications system of the invention canallocate resources to satisfy user requirements and achieve highefficiency and performance.

FIG. 3 is a block diagram of an embodiment of data processor 112 andmodulator 114 of system 110 in FIG. 1. The aggregate input data streamthat includes all data to be transmitted by system 110 is provided to ademultiplexer (DEMUX) 310 within data processor 112. Demultiplexer 310demultiplexes the input data stream into a number of (K) channel datastream, S₁ through S_(k). Each channel data stream may correspond to,for example, a signaling channel, a broadcast channel, a voice call, ora traffic data transmission. Each channel data stream is provided to arespective encoder 312 that encodes the data using a particular encodingscheme.

The encoding may include error correction coding or error detectioncoding, or both, used to increase the reliability of the link. Morespecifically, such encoding may include, for example, interleaving,convolutional coding, Turbo coding, Trellis coding, block coding (e.g.,Reed-Solomon coding), cyclic redundancy check (CRC) coding, and others.Turbo encoding is described in further detail in U.S. pat. applicationSer. No. 09/205,511, filed Dec. 4, 1998 entitled “Turbo Code InterleaverUsing Linear Congruential Sequences,” now U.S. Pat. No. 6,304,991 issuedon Oct. 16, 2001, and in a document entitled “The cdma2000 ITU-R RTTCandidate Submission,” hereinafter referred to as the IS-2000 standard,both of which are incorporated herein by reference.

The encoding can be performed on a per channel basis, i.e., on eachchannel data stream, as shown in FIG. 3. However, the encoding may alsobe performed on the aggregate input data stream, on a number of channeldata streams, on a portion of a channel data stream, across a set ofantennas, across a set of sub-channels, across a set of sub-channels andantennas, across each sub-channel, on each modulation symbol, or on someother unit of time, space, and frequency. The encoded data from encoders312 a through 312 k is then provided to a data processor 320 thatprocesses the data to generate modulation symbols.

In one implementation, data processor 320 assigns each channel datastream to one or more sub-channels, at one or more time slots, and onone or more antennas. For example, for a channel data streamcorresponding to a voice call, data processor 320 may assign onesub-channel on one antenna (if transmit diversity is not used) ormultiple antennas (if transmit diversity is used) for as many time slotsas needed for that call. For a channel data stream corresponding to asignaling or broadcast channel, data processor 320 may assign thedesignated sub-channel(s) on one or more antennas, again depending onwhether transmit diversity is used. Data processor 320 then assigns theremaining available resources for channel data streams corresponding todata transmissions. Because of the burstiness nature of datatransmissions and the greater tolerance to delays, data processor 320can assign the available resources such that the system goals of highperformance and high efficiency are achieved. The data transmissions arethus “scheduled” to achieve the system goals.

After assigning each channel data stream to its respective time slot(s),sub-channel(s), and antenna(s), the data in the channel data stream ismodulated using multi-carrier modulation. In an embodiment, OFDMmodulation is used to provide numerous advantages. In one implementationof OFDM modulation, the data in each channel data stream is grouped toblocks, with each block having a particular number of data bits. Thedata bits in each block are then assigned to one or more sub-channelsassociated with that channel data stream.

The bits in each block are then demultiplexed into separatesub-channels, with each of the sub-channels conveying a potentiallydifferent number of bits (i.e., based on C/I of the sub-channel andwhether MIMO processing is employed). For each of these sub-channels,the bits are grouped into modulation symbols using a particularmodulation scheme (e.g., M-PSK or M-QAM) associated with thatsub-channel. For example, with 16-QAM, the signal constellation iscomposed of 16 points in a complex plane (i.e., a+j*b), with each pointin the complex plane conveying 4 bits of information. In the MIMOprocessing mode, each modulation symbol in the sub-channel represents alinear combination of modulation symbols, each of which may be selectedfrom a different constellation.

The collection of L modulation symbols forms a modulation symbol vectorV of dimensionality L. Each element of the modulation symbol vector V isassociated with a specific sub-channel having a unique frequency or toneon which the modulation symbols are conveyed. The L modulation symbolsin the collection are all orthogonal to one another. At each time slotand for each antenna, the L modulation symbols corresponding to the Lsub-channels are combined into an OFDM symbol using an inverse fastFourier transform (IFFT). Each OFDM symbol includes data from thechannel data streams assigned to the L sub-channels.

OFDM modulation is described in further detail in a paper entitled“Multicarrier Modulation for Data Transmission: An Idea Whose Time HasCome,” by John A. C. Bingham, IEEE Communications Magazine, May 1990,which is incorporated herein by reference.

Data processor 320 thus receives and processes the encoded datacorresponding to K channel data streams to provide N_(T) modulationsymbol vectors, V₁ through V_(N) _(T) , one modulation symbol vector foreach transmit antenna. In some implementations, some of the modulationsymbol vectors may have duplicate information on specific sub-channelsintended for different transmit antennas. The modulation symbol vectorsV₁ through V_(N) _(T) are provided to modulators 114 a through 114 t,respectively.

In the embodiment shown in FIG. 3, each modulator 114 includes an IFFT330, cycle prefix generator 332, and an upconverter 334. IFFT 330converts the received modulation symbol vectors into their time-domainrepresentations called OFDM symbols. IFFT 330 can be designed to performthe IFFT on any number of sub-channels (e.g., 8, 16, 32, and so on). Inan embodiment, for each modulation symbol vector converted to an OFDMsymbol, cycle prefix generator 332 repeats a portion of the time-domainrepresentation of the OFDM symbol to form the transmission symbol forthe specific antenna. The cyclic prefix insures that the transmissionsymbol retains its orthogonal properties in the presence of multipathdelay spread, thereby improving performance against deleterious patheffects, as described below. The implementation of IFFT 330 and cycleprefix generator 332 is known in the art and not described in detailherein.

The time-domain representations from each cycle prefix generator 332(i.e., the transmission symbols for each antenna) are then processed byupconverter 334, converted into an analog signal, modulated to a RFfrequency, and conditioned (e.g., amplified and filtered) to generate anRF modulated signal that is then transmitted from the respective antenna116.

FIG. 3 also shows a block diagram of an embodiment of data processor320. The encoded data for each channel data stream (i.e., the encodeddata stream, X) is provided to a respective channel data processor 322.If the channel data stream is to be transmitted over multiplesub-channels and/or multiple antennas (without duplication on at leastsome of the transmissions), channel data processor 322 demultiplexes thechannel data stream into a number of (up to L·N_(T)) data sub-streams.Each data sub-stream corresponds to a transmission on a particularsub-channel at a particular antenna. In typical implementations, thenumber of data sub-streams is less than L·N_(T) since some of thesub-channels are used for signaling, voice, and other types of data. Thedata sub-streams are then processed to generate correspondingsub-streams for each of the assigned sub-channels that are then providedto combiners 324. Combiners 324 combine the modulation symbolsdesignated for each antenna into modulation symbol vectors that are thenprovided as a modulation symbol vector stream. The N_(T) modulationsymbol vector streams for the N_(T) antennas are then provided to thesubsequent processing blocks (i.e., modulators 114).

In a design that provides the most flexibility, best performance, andhighest efficiency, the modulation symbol to be transmitted at each timeslot, on each sub-channel, can be individually and independentlyselected. This feature allows for the best use of the available resourceover all three dimensions—time, frequency, and space. The number of databits transmitted by each modulation symbol may thus differ.

FIG. 4A is a block diagram of an embodiment of a channel data processor400 that can be used for processing one channel data steam. Channel dataprocessor 400 can be used to implement one channel data processor 322 inFIG. 3. The transmission of a channel data stream may occur on multiplesub-channels (e.g., as for data 1 in FIG. 2) and may also occur frommultiple antennas. The transmission on each sub-channel and from eachantenna can represent non-duplicated data.

Within channel data processor 400, a demultiplexer 420 receives anddemultiplexes the encoded data stream, X_(i), into a number ofsub-channel data streams, X_(i,1) through X_(i,M) , one sub-channel datastream for each sub-channel being used to transmit data. The datademultiplexing can be uniform or non-uniform. For example, if someinformation about the transmission paths is known (i.e., full CSI orpartial CSI is known), demultiplexer 420 may direct more data bits tothe sub-channels capable of transmitting more bps/Hz. However, if no CSIis known, demultiplexer 420 may uniformly directs approximately equalnumber of bits to each of the allocated sub-channels.

Each sub-channel data stream is then provided to a respective spatialdivision processor 430. Each spatial division processor 430 may furtherdemultiplex the received sub-channel data stream into a number of (up toN_(T)) data sub-streams, one data sub-stream for each antenna used totransmit the data. Thus, after demultiplexer 420 and spatial divisionprocessor 430, the encoded data stream X_(i) may be demultiplexed intoup to L·N_(T) data sub-streams to be transmitted on up to L sub-channelsfrom up to N_(T) antennas.

At any particular time slot, up to N_(T) modulation symbols may begenerated by each spatial division processor 430 and provided to N_(T)combiners 400 a through 440 t. For example, spatial division processor430 a assigned to sub-channel 1 may provide up to N_(T) modulationsymbols for sub-channel 1 of antennas 1 through N_(T). Similarly,spatial division processor 430 k assigned to sub-channel k may provideup to N_(T) symbols for sub-channel k of antennas 1 through N_(T). Eachcombiner 440 receives the modulation symbols for the L sub-channels,combines the symbols for each time slot into a modulation symbol vector,and provides the modulation symbol vectors as a modulation symbol vectorstream, V, to the next processing stage (e.g., modulator 114).

Channel data processor 400 may also be designed to provide the necessaryprocessing to implement the full-CSI or partial-CSI processing modesdescribed above. The CSI processing may be performed based on theavailable CSI information and on selected channel data streams,sub-channels, antennas, etc. The CSI processing may also be enabled anddisabled selectively and dynamically. For example, the CSI processingmay be enabled for a particular transmission and disabled for some othertransmissions. The CSI processing may be enabled under certainconditions, for example, when the transmission link has adequate C/I.

Channel data processor 400 in FIG. 4A provides a high level offlexibility. However, such flexibility is typically not needed for allchannel data streams. For example, the data for a voice call istypically transmitted over one sub-channel for the duration of the call,or until such time as the sub-channel is reassigned. The design of thechannel data processor can be greatly simplified for these channel datastreams.

FIG. 4B is a block diagram of the processing that can be employed forone channel data steam such as overhead data, signaling, voice, ortraffic data. A spatial division processor 450 can be used to implementone channel data processor 322 in FIG. 3 and can be used to support achannel data stream such as, for example, a voice call. A voice call istypically assigned to one sub-channel for multiple time slots (e.g.,voice 1 in FIG. 2) and may be transmitted from multiple antennas. Theencoded data stream, X_(j), is provided to spatial division processor450 that groups the data into blocks, with each block having aparticular number of bits that are used to generate a modulation symbol.The modulation symbols from spatial division processor 450 are thenprovided to one or more combiners 440 associated with the one or moreantennas used to transmit the channel data stream.

A specific implementation of a transmitter unit capable of generatingthe transmit signal shown in FIG. 2 is now described for a betterunderstanding of the invention. At time slot 2 in FIG. 2, control datais transmitted on sub-channel 1, broadcast data is transmitted onsub-channel 2, voice calls 1 and 2 are assigned to sub-channels 3 and 4,respectively, and traffic data is transmitted on sub-channels 5 through16. In this example, the transmitter unit is assumed to include fourtransmit antennas (i.e., N_(T)=4) and four transmit signals (i.e., fourRF modulated signals) are generated for the four antennas.

FIG. 5A is a block diagram of a portion of the processing units that canbe used to generate the transmit signal for time slot 2 in FIG. 2. Theinput data stream is provided to a demultiplexer (DEMUX) 510 thatdemultiplexes the stream into five channel data streams, S₁ through S₅,corresponding to control, broadcast, voice 1, voice 2, and data 1 inFIG. 2. Each channel data stream is provided to a respective encoder 512that encodes the data using an encoding scheme selected for that stream.

In this example, channel data streams S₁ through S₃ are transmittedusing transmit diversity. Thus, each of the encoded data streams X₁through X₃ is provided to a respective channel data processor 532 thatgenerates the modulation symbols for that stream. The modulation symbolsfrom each of the channel data processors 532 a through 532 c are thenprovided to all four combiners 540 a through 540 d. Each combiner 540receives the modulation symbols for all 16 sub-channels designated forthe antenna associated with the combiner, combines the symbols on eachsub-channel at each time slot to generate a modulation symbol vector,and provides the modulation symbol vectors as a modulation symbol vectorstream, V, to an associated modulator 114. As indicated in FIG. 5A,channel data stream S₁ is transmitted on sub-channel 1 from all fourantennas, channel data stream S₂ is transmitted on sub-channel 2 fromall four antennas, and channel data stream S₃ is transmitted onsub-channel 3 from all four antennas.

FIG. 5B is a block diagram of a portion of the processing units used toprocess the encoded data for channel data stream S₄. In this example,channel data stream S₄ is transmitted using spatial diversity (and nottransmit diversity as used for channel data streams S₁ through S₃). Withspatial diversity, data is demultiplexed and transmitted (concurrentlyin each of the assigned sub-channels or over different time slots) overmultiple antennas. The encoded data stream X₄ is provided to a channeldata processor 532 d that generates the modulation symbols for thatstream. The modulation symbols in this case are linear combinations ofmodulation symbols selected from symbol alphabets that correspond toeach of the eigenmodes of the channel. In this example, there are fourdistinct eigenmodes, each of which is capable of conveying a differentamount of information. As an example, suppose eigenmode 1 has a C/I thatallows 64-QAM (6 bits) to be transmitted reliably, eigenmode 2 permits16-QAM (4 bits), eigenmode 3 permits QPSK (2 bits) and eigenmode 4permits BPSK (1 bit) to be used. Thus, the combination of all foureigenmodes allows a total of 13 information bits to be transmittedsimultaneously as an effective modulation symbol on all four antennas inthe same sub-channel. The effective modulation symbol for the assignedsub-channel on each antenna is a linear combination of the individualsymbols associated with each eigenmode, as described by the matrixmultiply given in equation (1) above.

FIG. 5C is a block diagram of a portion of the processing units used toprocess channel data stream S₅. The encoded data stream X₅ is providedto a demultiplexer (DEMUX) 530 that demultiplexes the stream X₅ intotwelve sub-channel data streams, X_(5,5) through X_(5,16), onesub-channel data stream for each of the allocated sub-channels 5 through16. Each sub-channel data stream is then provided to a respectivesub-channel data processor 536 that generates the modulation symbols forthe associated sub-channel data stream. The sub-channel symbol streamfrom sub-channel data processors 536 a through 5361 are then provided todemultiplexers 538 a through 5381, respectively. Each demultiplexer 538demultiplexes the received sub-channel symbol stream into four symbolsub-streams, with each symbol sub-stream corresponding to a particularsub-channel at a particular antenna. The four symbol sub-streams fromeach demultiplexer 538 are then provided to the four combiners 540 athrough 540 d.

In the embodiment described for FIG. 5C, a sub-channel data stream isprocessed to generate a sub-channel symbol stream that is thendemultiplexed into four symbol sub-streams, one symbol sub-stream for aparticular sub-channel of each antenna. This implementation is adifferent from that described for FIG. 4A. In the embodiment describedfor FIG. 4A, the sub-channel data stream designated for a particularsub-channel is demultiplexed into a number of data sub-streams, one datasub-stream for each antenna, and then processed to generate thecorresponding symbol sub-streams. The demultiplexing in FIG. 5C isperformed after the symbol modulation whereas the demultiplexing in FIG.4A is performed before the symbol modulation. Other implementations mayalso be used and are within the scope of the present invention.

Each combination of sub-channel data processor 536 and demultiplexer 538in FIG. 5C performs in similar manner as the combination of sub-channeldata processor 532 d and demultiplexer 534 d in FIG. 5B. The rate ofeach symbol sub-stream from each demultiplexer 538 is, on the average, aquarter of the rate of the symbol stream from the associated channeldata processor 536.

FIG. 6 is a block diagram of an embodiment of a receiver unit 600,having multiple receive antennas, which can be used to receive one ormore channel data streams. One or more transmitted signals from one ormore transmit antennas can be received by each of antennas 610 a through610 r and routed to a respective front-end processor 612. For example,receive antenna 610 a may receive a number of transmitted signals from anumber of transmit antennas, and receive antenna 610 r may similarlyreceive multiple transmitted signals. Each front-end processor 612conditions (e.g., filters and amplifies) the received signal,downconverts the conditioned signal to an intermediate frequency orbaseband, and samples and quantizes the downconverted signal. Eachfront-end processor 612 typically further demodulates the samplesassociated with the specific antenna with the received pilot to generate“coherent” samples that are then provided to a respective FFT processor614, one for each receive antenna.

Each FFT processor 614 generates transformed representations of thereceived samples and provides a respective stream of modulation symbolvectors. The modulation symbol vector streams from FFT processors 614 athrough 614 r are then provided to demultiplexer and combiners 620,which channelizes the stream of modulation symbol vectors from each FFTprocessor 614 into a number of (up to L) sub-channel symbol streams. Thesub-channel symbol streams from all FFT processors 614 are thenprocessed, based on the (e.g., diversity or MIMO) communications modeused, prior to demodulation and decoding.

For a channel data stream transmitted using the diversity communicationsmode, the sub-channel symbol streams from all antennas used for thetransmission of the channel data stream are presented to a combiner thatcombines the redundant information across time, space, and frequency.The stream of combined modulation symbols are then provided to a(diversity) channel processor 630 and demodulated accordingly.

For a channel data stream transmitted using the MIMO communicationsmode, all sub-channel symbol streams used for the transmission of thechannel data stream are presented to a MIMO processor thatorthogonalizes the received modulation symbols in each sub-channel intothe distinct eigenmodes. The MIMO processor performs the processingdescribed by equation (2) above and generates a number of independentsymbol sub-streams corresponding to the number of eigenmodes used at thetransmitter unit. For example, MIMO processor can perform multiplicationof the received modulation symbols with the left eigenvectors togenerate post-conditioned modulation symbols, which correspond to themodulation symbols prior to the full-CSI processor at the transmitterunit. The (post-conditioned) symbol sub-streams are then provided to a(MIMO) channel processor 630 and demodulated accordingly. Thus, eachchannel processor 630 receives a stream of modulation symbols (for thediversity communications mode) or a number of symbol sub-streams (forthe MIMO communications mode). Each stream or sub-stream of modulationsymbols is then provided to a respective demodulator (DEMOD) thatimplements a demodulation scheme (e.g., M-PSK, M-QAM, or others) that iscomplementary to the modulation scheme used at the transmitter unit forthe sub-channel being processed. For the MIMO communications mode, thedemodulated data from all assigned demodulators may then be decodedindependently or multiplexed into one channel data stream and thendecoded, depending upon the coding and modulation method employed at thetransmitter unit. For both the diversity and MIMO communications modes,the channel data stream from channel processor 630 may then provided toa respective decoder 640 that implements a decoding scheme complementaryto that used at the transmitter unit for the channel data stream. Thedecoded data from each decoder 540 represents an estimate of thetransmitted data for that channel data stream.

FIG. 6 represents one embodiment of a receiver unit. Other designs canbe contemplated and are within the scope of the present invention. Forexample, a receiver unit may be designed with only one receive antenna,or may be designed capable of simultaneous processing multiple (e.g.,voice, data) channel data streams.

As noted above, multi-carrier modulation is used in the communicationssystem of the invention. In particular, OFDM modulation can be employedto provide a number of benefits including improved performance in amultipath environment, reduced implementation complexity (in a relativesense, for the MIMO mode of operation), and flexibility. However, othervariants of multi-carrier modulation can also be used and are within thescope of the present invention.

OFDM modulation can improve system performance due to multipath delayspread or differential path delay introduced by the propagationenvironment between the transmitting antenna and the receiver antenna.The communications link (i.e., the RF channel) has a delay spread thatmay potentially be greater than the reciprocal of the system operatingbandwidth, W. Because of this, a communications system employing amodulation scheme that has a transmit symbol duration of less than thedelay spread will experience inter-symbol interference (ISI). The ISIdistorts the received symbol and increases the likelihood of incorrectdetection.

With OFDM modulation, the transmission channel (or operating bandwidth)is essentially divided into a (large) number of parallel sub-channels(or sub-bands) that are used to communicate the data. Because each ofthe sub-channels has a bandwidth that is typically much less than thecoherence bandwidth of the communications link, ISI due to delay spreadin the link is significantly reduced or eliminated using OFDMmodulation. In contrast, most conventional modulation schemes (e.g.,QPSK) are sensitive to ISI unless the transmission symbol rate is smallcompared to the delay spread of the communications link.

As noted above, cyclic prefix can be used to combat the deleteriouseffects of multipath. A cyclic prefix is a portion of an OFDM symbol(usually the front portion, after the IFFT) that is wrapped around tothe back of the symbol. The cyclic prefix is used to retainorthogonality of the OFDM symbol, which is typically destroyed bymultipath.

As an example, consider a communications system in which the channeldelay spread is less than 10 μsec. Each OFDM symbol has appended onto ita cyclic prefix that insures that the overall symbol retains itsorthogonal properties in the presence of multipath delay spread. Sincethe cyclic prefix conveys no additional information, it is essentiallyoverhead. To maintain good efficiency, the duration of the cyclic prefixis selected to be a small fraction of the overall transmission symbolduration. For the above example, using a 5% overhead to account for thecyclic prefix, a transmission symbol duration of 200 μsec is adequatefor a 10 μsec maximum channel delay spread. The 200 μsec transmissionsymbol duration corresponds to a bandwidth of 5 kHz for each of thesub-bands. If the overall system bandwidth is 1.2288 MHz, 250sub-channels of approximately 5 kHz can be provided. In practice, it isconvenient for the number of sub-channels to be a power of two. Thus, ifthe transmission symbol duration is increased to 205 μsec and the systembandwidth is divided into M=256 sub-bands, each sub-channel will have abandwidth of 4.88 kHz.

In certain embodiments of the invention, OFDM modulation can reduce thecomplexity of the system. When the communications system incorporatesMIMO technology, the complexity associated with the receiver unit can besignificant, particularly when multipath is present. The use of OFDMmodulation allows each of the sub-channels to be treated in anindependent manner by the MIMO processing employed. Thus, OFDMmodulation can significantly simplify the signal processing at thereceiver unit when MIMO technology is used.

OFDM modulation can also afford added flexibility in sharing the systembandwidth, W, among multiple users. Specifically, the availabletransmission space for OFDM symbols can be shared among a group ofusers. For example, low rate voice users can be allocated a sub-channelor a fraction of a sub-channel in OFDM symbol, while the remainingsub-channels can be allocated to data users based on aggregate demand.In addition, overhead, broadcast, and control data can be conveyed insome of the available sub-channels or (possibly) in a portion of asub-channel.

As described above, each sub-channel at each time slot is associatedwith a modulation symbol that is selected from some alphabet such asM-PSK or M-QAM. In certain embodiments, the modulation symbol in each ofthe L sub-channels can be selected such that the most efficient use ismade of that sub-channel. For example, sub-channel 1 can be generatedusing QPSK, sub-channel 2 can be generate using BPSK, sub-channel 3 canbe generated using 16-QAM, and so on. Thus, for each time slot, up to Lmodulation symbols for the L sub-channels are generated and combined togenerate the modulation symbol vector for that time slot.

One or more sub-channels can be allocated to one or more users. Forexample, each voice user may be allocated a single sub-channel. Theremaining sub-channels can be dynamically allocated to data users. Inthis case, the remaining sub-channels can be allocated to a single datauser or divided among multiple data users. In addition, somesub-channels can be reserved for transmitting overhead, broadcast, andcontrol data. In certain embodiments of the invention, it may bedesirable to change the sub-channel assignment from (possibly)modulation symbol to symbol in a pseudo-random manner to increasediversity and provide some interference averaging.

In a CDMA system, the transmit power on each reverse link transmissionis controlled such that the required frame error rate (FER) is achievedat the base station at the minimal transmit power, thereby minimizinginterference to other users in the system. On the forward link of theCDMA system, the transmit power is also adjusted to increase systemcapacity.

In the communications system of the invention, the transmit power on theforward and reverse links can be controlled to minimize interference andmaximize system capacity. Power control can be achieved in variousmanners. For example, power control can be performed on each channeldata stream, on each sub-channel, on each antenna, or on some other unitof measurements. When operating in the diversity communications mode, ifthe path loss from a particular antenna is great, transmission from thisantenna can be reduced or muted since little may be gained at thereceiver unit. Similarly, if transmission occurs over multiplesub-channels, less power may be transmitted on the sub-channel(s)experiencing the most path loss.

In an implementation, power control can be achieved with a feedbackmechanism similar to that used in the CDMA system. Power controlinformation can be sent periodically or autonomously from the receiverunit to the transmitter unit to direct the transmitter unit to increaseor decrease its transmit power. The power control bits may be generatedbased on, for example, the BER or FER at the receiver unit.

FIG. 7 shows plots that illustrate the spectral efficiency associatedwith some of the communications modes of the communications system ofthe invention. In FIG. 7, the number of bits per modulation symbol for agiven bit error rate is given as a function of C/I for a number ofsystem configurations. The notation N_(T)×N_(R) denotes thedimensionality of the configuration, with N_(T)=number of transmitantennas and N_(R)=number of receive antennas. Two diversityconfigurations, namely 1×2 and 1×4, and four MIMO configurations, namely2×2, 2×4, 4×4, and 8×4, are simulated and the results are provided inFIG. 7.

As shown in the plots, the number of bits per symbol for a given BERranges from less than 1 bps/Hz to almost 20 bps/Hz. At low values ofC/I, the spectral efficiency of the diversity communications mode andMIMO communications mode is similar, and the improvement in efficiencyis less noticeable. However, at higher values of C/I, the increase inspectral efficiency with the use of the MIMO communications mode becomesmore dramatic. In certain MIMO configurations and for certainconditions, the instantaneous improvement can reach up to 20 times.

From these plots, it can be observed that spectral efficiency generallyincreases as the number of transmit and receive antennas increases. Theimprovement is also generally limited to the lower of N_(T) and N_(R).For example, the diversity configurations, 1×2 and 1×4, bothasymptotically reach approximately 6 bps/Hz.

In examining the various data rates achievable, the spectral efficiencyvalues given in FIG. 7 can be applied to the results on a sub-channelbasis to obtain the range of data rates possible for the sub-channel. Asan example, for a subscriber unit operating at a C/I of 5 dB, thespectral efficiency achievable for this subscriber unit is between 1bps/Hz and 2.25 bps/Hz, depending on the communications mode employed.Thus, in a 5 kHz sub-channel, this subscriber unit can sustain a peakdata rate in the range of 5 kbps to 10.5 kbps. If the C/I is 10 dB, thesame subscriber unit can sustain peak data rates in the range of 10.5kbps to 25 kbps per sub-channel. With 256 sub-channels available, thepeak sustained data rate for a subscriber unit operating at 10 dB C/I isthen 6.4 Mbps. Thus, given the data rate requirements of the subscriberunit and the operating C/I for the subscriber unit, the system canallocate the necessary number of sub-channels to meet the requirements.In the case of data services, the number of sub-channels allocated pertime slot may vary depending on, for example, other traffic loading.

The reverse link of the communications system can be designed similar instructure to the forward link. However, instead of broadcast and commoncontrol channels, there may be random access channels defined inspecific sub-channels or in specific modulation symbol positions of theframe, or both. These may be used by some or all subscriber units tosend short requests (e.g., registration, request for resources, and soon) to the central station. In the common access channels, thesubscriber units may employ common modulation and coding. The remainingchannels may be allocated to separate users as in the forward link. Inan embodiment, allocation and de-allocation of resources (on both theforward and reverse links) are controlled by the system and communicatedon the control channel in the forward link.

One design consideration for on the reverse link is the maximumdifferential propagation delay between the closest subscriber unit andthe furthest subscriber unit. In systems where this delay is smallrelative to the cyclic prefix duration, it may not be necessary toperform correction at the transmitter unit. However, in systems in whichthe delay is significant, the cyclic prefix can be extended to accountfor the incremental delay. In some instances, it may be possible to makea reasonable estimate of the round trip delay and correct the time oftransmit so that the symbol arrives at the central station at thecorrect instant. Usually there is some residual error, so the cyclicprefix may also further be extended to accommodate this residual error.

In the communications system, some subscriber units in the coverage areamay be able to receive signals from more than one central station. Ifthe information transmitted by multiple central stations is redundant ontwo or more sub-channels and/or from two or more antennas, the receivedsignals can be combined and demodulated by the subscriber unit using adiversity-combining scheme. If the cyclic prefix employed is sufficientto handle the differential propagation delay between the earliest andlatest arrival, the signals can be (optimally) combined in the receiverand demodulated correctly. This diversity reception is well known inbroadcast applications of OFDM. When the sub-channels are allocated tospecific subscriber units, it is possible for the same information on aspecific sub-channel to be transmitted from a number of central stationsto a specific subscriber unit. This concept is similar to the softhandoff used in CDMA systems.

The communications system described above can be used for variousapplications and to provide various services. Such services may include,for example, real time services, non-real time services, or both realtime and non-real time services multiplexed together. The services to besupported by the communications system can be defined and categorized invarious manners (e.g., by the quality of service (QoS) associated withthe services). As an example, the supported services can be categorizedinto three types defined as follow:

-   -   Full duplex real time (FDRT) services—services that require        short one-way delay (e.g., voice);    -   Half duplex real time (HDRT) services—services that can tolerate        longer one-way delay, as long as the delay does not vary by a        large amount (e.g., video, audio); and    -   Non-real time (NRT) services—services that are not quite as        sensitive to delay (e.g., packet data).        Additional and/or different services can also be supported and        are within the scope of the invention. For example, broadcast        services, paging services, and others can be supported.        Similarly, additional and/or different types of services can        also be defined and are within the scope of the invention.

Once the services have been defined and categorized, they can bemultiplexed in numerous manners. For example, HDRT and NRT services canbe multiplexed in a single data transmission, with the HDRT servicesbeing given higher priority. FDRT services can also be multiplexed withHDRT and NRT services, possibly using a different multiplexing scheme.Various multiplexing schemes can be used to transmit the supportedservices. Some of these schemes are described in further detail below.

FIG. 8A is a diagram of an embodiment of a structure that can be used totransmit various types of services. In this embodiment, the supportedservices are multiplexed and transmitted in slots (only one slot isshown in FIG. 8A for simplicity). Each slot covers N OFDM symbols 810 athrough 810 n, where N can be defined to be any integer. In anembodiment, each slot is further divided into a number of partitions 802(two is shown in FIG. 8A for simplicity). Each partition 802 can includeany number of OFDM symbols and can be used to support any type ofservices. For example, partition 802 a can be used to support FDRTservices (e.g., voice), and partition 802 b can be used to support HDRTand/or NRT services (e.g., packet data). Other structures can also beimplemented and are within the scope of the invention.

The partition used to support FDRT services and the partition used tosupport HDRT and NRT services can each be shared by multiple users. Thesharing of a partition can be achieved by various multiplexing schemes.For example, the sharing can be achieved by:

-   -   multiplexing multiple users on different (orthogonal) OFDM        tones;    -   multiplexing multiple users on common OFDM tones using Walsh        codes;    -   multiplexing multiple users on common OFDM symbols using packet        switching; and    -   assigning multiple users to their respective OFDM symbols.        These multiplexing schemes are described in further detail        below. Other multiplexing schemes can also be defined and are        within the scope of the invention.

FIG. 9 is a block diagram of an embodiment of a data processor 912 and amodulator 914 that can be used to multiplex multiple users on orthogonalOFDM tones. Channel data streams S₁ through S_(K) can be used to carrydata for users 1 through K, respectively. Additional channel datastreams (e.g., S_(L)) can be used to carry data for control, signaling,broadcast, and other overhead channels. Each channel data stream isprovided to a respective encoder 922 that codes the received data with aparticular coding scheme selected for that channel. For example, thecoding scheme can include convolutional coding, Turbo coding, or nocoding at all. The encoded data streams X₁ through X_(L) from encoders922 a through 922 l are then provided to respective multipliers 924 athrough 924 l, which also receive respective scaling factors G₁ throughG_(L). Each multiplier 924 scales the received data stream with thereceived scaling factor to provide power adjustment for the data stream.

The scaled data streams from multipliers 924 a through 9241 are thenprovided to a parallel to serial converter (P/S) 926 that multiplexesthe received data streams into a combined data stream. A symbol mappingelement 928 then receives the combined data stream and interleaves(i.e., reorders) the data in the stream to provide temporal diversity.Symbol mapping element 928 further maps the data in each received datastream to the tones assigned to the data stream, as described below. Theoutput from symbol mapping element 928 is a stream of modulation symbolvectors V, which is provided to modulator 914.

Within modulator 914, an IFFT 930 receives and converts the modulationsymbol vectors V into their time-domain representations called OFDMsymbols. In an embodiment, for each modulation symbol vector convertedto an OFDM symbol, cycle prefix generator 932 repeats a portion of thetime-domain representation of the OFDM symbol to form a transmissionsymbol. The cyclic prefix ensures that the transmission symbol retainsits orthogonal properties in the presence of multipath delay spread,thereby improving performance against deleterious path effects, asdescribed above. The transmission symbols from cycle prefix generator932 are then processed by upconverter 934, converted into an analogsignal, modulated to a RF frequency, and conditioned (e.g., amplifiedand filtered) to generate an RF modulated signal that is thentransmitted from an antenna 916.

In an embodiment, symbol mapping element 928 maps the symbols for eachchannel data stream (e.g., each user) to a set of tones that areassigned to the channel. Referring back to FIG. 8A, each partitionincludes a number of OFDM symbols and, referring back to FIG. 1, eachOFDM symbol includes a number of tones transmitted on a number ofsub-channels. Thus, a number of tones in each partition are availablefor transmitting the channel data streams.

In an embodiment, the available tones in each multi-user partition aregrouped to a number of sets of tones. Each set of tones is referred toas a “circuit” and is assigned to a particular channel. The L channeldata streams can thus be assigned to L circuits. A particular channeldata stream may also be assigned multiple circuits in one or morepartitions (e.g., one or more circuits in partition 802 a and one ormore circuits in partition 802 b). Also, a user may be assigned withmultiple channel data streams or a channel data stream may be sharedbetween multiple users. Multiple channel data streams may also share thesame circuit.

Each of the circuits can be defined to include any number of tones. Moretones may be allocated to high rate circuits and fewer tones may beallocated to low rate circuits. Also, each circuit may be assigned withany tone from any OFDM symbol. The tones of each OFDM symbol may thus beassigned to one or more circuits. For improved frequency and temporaldiversity, the tones for each circuit can be selected such thatdifferent tones from different OFDM symbols are assigned to the circuit.For example, a particular circuit may be assigned with tone 1 of thefirst OFDM symbol, tone 2 of the second OFDM symbol, and so on.

Different types of circuits can be defined and used for different typesof services. For example, a first circuit type can be defined to includedifferent tones from different OFDM symbols, and a second circuit typecan be defined to include all tones from one or more OFDM symbols. Thefirst circuit type (e.g., in partition 802 a) can be used to supportFDRT services, and the second circuit type (e.g., in partition 802 b)can be used to support HDRT and NRT services. With the second circuittype, each OFDM symbol can be assigned to a particular HDRT or NRT user.

The circuits may be defined as static sets of tones or may bedynamically configurable. For example, the communications system maydefine each of the circuits available for assignment and thereafterinforms each user terminal of the assigned circuit and its definition(e.g., during session initiation). The dynamic definition of thecircuits allows for customization of the circuits to match the servicesbeing supported, and can result in improved utilization of the availableresources.

The circuits may be defined to be equal-size, with each circuit havingthe capacity to carry a particular number of bits. Alternatively, thecircuits may be designed to have different sizes. The circuit sizes maybe based on the statistics of the users in the communications system orsome other criteria. For example, lower rate circuits can be defined ifmore users are utilizing low rate services. Alternatively oradditionally, the circuits may be defined based on the particularservices being supported and/or user requirements. High rate circuitswith more tones can be assigned to high rate services, and low ratecircuits with fewer tones (or transmitted less frequently) can beassigned to low rate services.

The circuits are typically defined a priori, prior to a call processing(e.g., circuit # 0 include tones x, y, z, and so on). One or morecircuits can be assigned for a particular communications session, andthe assigned circuits can be provided to a user terminal, for example,via control channel system parameter messages that describe how theforward link is configured.

FIG. 8B is a diagram of a specific embodiment of a packet structure 820,which can be used for transmitting data in the communications system.Packet structure 820 includes a packet type field 822, a circuitidentifier field 824, and a data field 826. Packet type field 822 isused to signal whether the circuit assigned to a user terminal will bechanged for the next packet to be transmitted to the terminal. Packettype field 822 can also be used to inform the user terminal of a changein transmission scheme. For example, for voice service, packet typefield 822 can be used to signal a change from voice activity to silenceand vice versa, each of which may be associated with a differenttransmission scheme, as described below. Table 1 shows a specificdefinition for packet type field 822.

TABLE 1 Packet Type Value Definition 00 No change in circuit 01 Changeto new circuit 10 Transition to silence 11 Transition to activity

If packet type field 822 indicates that the circuit to be used for thenext packet will be different, then circuit identifier field 824 can beused to identify the particular circuit to be used for the next packet.Circuit identifier field 824 can provide the identity of the new circuitas well as other information (e.g., the transmission scheme), asdescribed below. Circuit identifier field 824 is typically used only ifthere is information to be sent (e.g., for a change in circuit ortransmission scheme). Otherwise, circuit identifier field 824 is a nullfield.

Data field 826 can be used to carry the payload (e.g., data) for thetransmission. Data field 826 may also be used to carry other informationsuch as, for example, control data, CRC bits, and so on.

Packet type field 822 can be implemented with few bits (e.g., 2 bits),and circuit identifier field 824 can be implemented with a small numberof bits (e.g., 8-10 bits). The remaining bits in each packet can be usedfor data field 826. This results in an efficient packet format in whichfew overhead bits are required.

Each packet can be dimensioned to fit into one circuit in one slot.However, a packet may also be segmented and transmitted using multiplecircuits over one or more slots. The size of the packet can be selectedfor efficient data transmission. For services that can tolerate longerprocessing delays, low rate transmissions can be collected and assembledinto a larger packet (e.g., 20 msec or 40 msec of data) that can be moreefficiently processed and transmitted.

Packet structure 820 supports inband signaling of a change in circuitassignment and the identity of the new circuit. This information canalso be provided via a control channel. The user terminal would thenprocess the control channel to receive changes in circuit assignment.Other signaling schemes to communicate circuit information to the userterminal can also be used and are within the scope of the invention.

For voice data, various transmission schemes can be used to reduce theamount of transmission during periods of silence (e.g., pauses) or lowactivity. During silence periods, “comfort noise” is typically sent tothe user terminal. This noise can be sent at a lower rate than for fullspeech. In one transmission scheme, during silence periods, a full ratepacket is sent every X slots (e.g., X can be 4, 8, 16 or some othervalue). This scheme allows up to X users to share the same circuitduring silence periods, with each user being assigned to one of the Xslots. In another transmission scheme, a low rate circuit that includesfewer tones can be used to send comfort noise. This low rate circuit canbe sent every slot, or every few slots (but typically more frequent thanevery X slots). In yet another transmission scheme, a full rate circuitcan be sent for comfort noise but at a lower rate (e.g., using a lowrate code). This full rate circuit is typically the same as that usedfor active speech. The transmit power can be reduced for this full ratecircuit during silence periods. Various other transmission schemes tosend comfort noise (or other data) at lower bit rate can also becontemplated and are within the scope of the invention.

The transmission schemes described above to reduce the amount oftransmission during silence periods can also be used for any data beingsent at less than full rate. For example, speech activity having lowfrequency contents may be represented using fewer bits and can be sentusing a low rate circuit or a full rate circuit transmitted lessfrequently. The user terminal can be informed accordingly whenever achange in circuit and/or transmission scheme is about to be made.

Packet format 820 supports the use of different circuits for voiceactivity and silence. When a user changes state from activity tosilence, packet type field 822 for the user's packet can beappropriately set to inform the user terminal to use the circuitidentified in circuit identifier field 824 for the next (e.g., noise)packet. Circuit identifier field 824 can also identify the particularslot used to carry comfort noise for this user terminal. Thereafter,during periods of silence, the noise packet can be sent every X slots(in one transmission scheme) to update the comfort noise that is playedat the terminal. In this way, each circuit used for silence can beshared by up to X users.

In a complementary manner, a full rate circuit is requested when a userchanges state from silence to activity. A scheduler receives the requestand assigns to the user a full rate circuit selected from a pool ofavailable full rate circuits. The identity of the assigned circuit issent to the user terminal in the next packet.

If the pool of available full rate circuits is empty, speech clippingcan occur until a circuit becomes available. The probability of speechclipping can be reduced by properly regulating the number of calls thatare connected, which is a parameter that can be adjusted by a calladmission policy if speech clipping is detected. If a large number ofusers are multiplexed together, the statistical multiplexing gain fromvoice activity is greater and the probability of speech clipping isreduced. A channel and circuit assignment protocol can be designed tominimize the probability of speech clipping without significantlyreducing the statistical multiplexing gain from voice activity.

Various signaling schemes can be used to signal to the user terminalthat speech activity has changed from silence to active. In one scheme,the signaling is achieved inband using, for example, packet type field822 in the comfort noise packet. As noted above, the noise packet may besent every X^(th) slots for some transmission schemes. To reduce thesignaling delays during silence periods, smaller noise packets can besent at a higher rate using smaller circuits. In another signalingscheme, a control channel can be used to inform the user terminal thattransition to full rate voice has occurred and to send the identity ofthe circuit that will be used for the next full rate packet. For thissignaling scheme, terminals that are in silence periods monitor thecontrol channel to receive the circuit information.

Various mechanisms can be implemented to ensure that changes to newcircuits are properly achieved. In one mechanism, the user terminalsends an acknowledgment to the base station whenever it receives apacket that contains a circuit change. To reduce the amount of overhead,the terminal can send a single bit to the base station after receipt ofa circuit change packet. This acknowledgment bit informs the basestation that the terminal has successfully decoded the previous packetand is ready to receive data using the new circuit. The base station cancontinue to transmit using the old circuit until it receives theacknowledgment. Upon receipt of the acknowledgment, the base stationtransmits using the new circuit, and the old circuit is placed back inthe pool of available circuits.

Several schemes can be used to handle false acknowledgments of circuitchanges that may result for various reasons. For example, a falseacknowledgment may result from a user terminal decoding the packet inerror and not sending an acknowledgment bit but the base station falselydetecting a transmission of the acknowledgment bit. In this case, thebase station starts transmitting on the new circuit while the terminalcontinues decoding the old circuit. A false acknowledgment may alsoresult from the user terminal properly decoding the packet and sendingan acknowledgment bit but the base station failing to detect theacknowledgment. In this case, the base station continues to transmit onthe old circuit while the terminal starts decoding the new circuit.

The probability of false acknowledgment can be reduced by using anenhanced acknowledgment protocol. For example, the acknowledgment bitcan be coded such that transmission error can be detected and/orcorrected. A recovery scheme can also be implemented whereby the userterminal informs the base station whenever it loses the forward link(e.g., as a result of the terminal decoding the new circuit while thebase station transmits on the old circuit, or vice versa). As part ofthe recovery scheme, the base station can send a circuit assignmentmessage to the terminal on a control channel whenever it receives achannel lost message. The terminal can then restart using theinformation included in the circuit assignment message. Other mechanismsto ensure that changes in circuits are properly implemented can bedesigned and are within the scope of the invention.

As noted above, a partition can be shared by multiplexing multiple userson the same OFDM symbols using packet switching. In this multiplexingscheme, each packet includes an identification of the specific user forwhich the packet is intended. Each packet can be transmitted, forexample, using one of the circuits described above. However, in thisscheme, the circuits are not individually assigned to the users.Instead, each user terminal processes all transmitted packets, extractsthe user identification in each packet, decodes packet directed towardthe terminal, and ignores remaining packets. Circuits of different sizescan be defined and used to efficiently transmit data.

FIG. 8C is a diagram of a specific embodiment of a packet structure 840,which can be used for transmitting user directed data. Packet structure840 includes a user identification (ID) field 842 and a data field 844.User ID field 842 includes the identity of the specific user for whichthe packet is destined and data field 844 contains the packet payload(e.g., the data). The user ID can be assigned to each user, for example,during session initiation.

User ID field 842 can be implemented as a preamble using a coding schemethat is different than that used for data field 844. For example, theuser ID can be a particular Walsh sequence or PN offset assigned to theuser terminal. This allows the user terminal to quickly determinewhether the packet is intended for it. Alternatively, the user ID can beimplemented as a coded sequence.

An 8-bit user ID field 842 can support up to 256 users. For a full ratepacket, the user ID overhead does not significantly impact theefficiency of the transmission. For lower rate packets, the overhead canbe a larger portion of the packet and efficiency may be compromised. Theoverhead for lower rate packets can be reduced by accumulating andtransmitting lower rate data in full rate packets that may be sent lessfrequently.

FIG. 10 is a block diagram of an embodiment of a data processor 1012 anda modulator 1014 that can be used to multiplex multiple users on thesame OFDM tones using orthogonal (e.g., Walsh) codes. Similar to FIG. 9,channel data streams S₁ through S_(L) can be used to carry data forusers and for control, signaling, broadcast, and other overheadchannels. Each channel data stream is provided to a respective encoder1022 that codes the received data with a particular coding schemeselected for that channel. The encoded data streams X₁ through X_(L)from encoders 1022 a through 1022 l are then provided to respectivemultipliers 1024 a through 1024 l, which also receive respective scalingfactors G₁ through G_(L). Each multiplier 1024 scales the received datastream with the received scaling factor to provide power control for thedata stream.

The scaled data streams from multipliers 1024 a through 1024 l are thenprovided to respective multipliers 1026 a through 1026 l, which alsoreceive respective Walsh sequences W₁ through W_(L). Each multiplier1026 covers the received data stream with the received Walsh sequence toprovide a covered data stream. The covered data streams from multipliers1026 a through 1026 l are provided to, and combined by a summer 1027 togenerate a combined data stream. A symbol mapping element 1028 receivesthe combined data stream and interleaves the data in the stream toprovide temporal diversity. The output from symbol mapping element 1028is a stream of modulation symbol vectors V, which is then provided tomodulator 1014.

Modulator 1014 includes an IFFT 1030, a cyclic prefix generator 1032,and an upconverter 1034 that operate in similar manner as IFFT 930,cyclic prefix generator 932, and upconverter 934, respectively, in FIG.9. Modulator 1014 generates an RF modulated signal that is transmittedfrom an antenna 1016.

In the embodiment shown in FIG. 10, the data for each user is coveredwith a respective Walsh sequence and transmitted over common tones.These tones carry data associated with one or more users. For multipleusers, orthogonality for the user data is maintained through the use ofthe Walsh sequences.

In a specific embodiment, the length of the Walsh sequences is matchedto the number of tones for each OFDM symbol. For example, Walshsequences of length 128 can be used for OFDM symbols having 128 tones.The 128 chips of each Walsh sequence can be transmitted on the 128 tonesof one OFDM symbol. However, other Walsh sequence lengths can also beused and are within the scope of the invention. Moreover, each Walshsequence can be mapped to multiple OFDM symbols or a portion of one OFDMsymbol, and these variations are within the scope of the invention. Forexample, if the Walsh sequences have length of 64 and each OFDM symbolhas 128 tones, then two sets of Walsh sequences can be mapped to eachOFDM symbol.

Various modulation schemes can be used to modulate ODFM symbols thathave been covered. These modulation schemes include QPSK, QAM, andothers.

At a user terminal, the tones are processed and decovered with theparticular Walsh sequence assigned to that terminal. Since the data formultiple users has been covered with orthogonal Walsh sequences, thedata previously covered with the particular Walsh sequence can berecovered by decovering with the same Walsh sequence. The datapreviously covered with other Walsh sequences is orthogonal and(ideally) sums to zero in the decovering.

If the Walsh covered data (i.e., the Walsh sequences) is transmittedacross multiple tones of the OFDM symbol, orthogonality of the Walshsequences may be diminished if the tones fade independently. This mayoccur, for example, with frequency selective fading. If the frequencyresponse of the transmission channel is not flat, channel equalizationmay be used to regain orthogonality. Equalization can be achieved bydetermining the channel gain for each tone in the OFDM symbol and usingthe determined channel gains to equalize the channel and make itapproximately flat. For example, if a particular tone has a channel lossof Y dB from a nominal value, that tone can be boosted by Y dB by theuser terminal. In this manner, orthogonality may be preserved in thepresence of frequency selective fading.

Since multiple users share the same tones in this multiplexing scheme,the transmit power for each user can be controlled to efficientlyutilize the available resource. The transmit power for users havinghigher signal-to-noise-plus-interference (Eb/Io) ratios can be reducedwhile maintaining a particular level of performance. The saving intransmit power can then be used for some other users. Power control canbe achieved, for example, using a scheme similar to that used in theIS-95 CDMA system whereby a user terminal sends a power control command(e.g., a frame-erasure-bit) to the base station, which then adjusts ittransmit power to this terminal accordingly.

The multiplexing schemes described above can be used for variousapplications. For example, these schemes can be used for mobile, fixed,and other applications.

For fixed application, a directional antenna can be used at the basestation for forward link transmissions, and two receive antennas can beprovided at the user terminal to achieve receive diversity. Thisconfiguration can provide a high carrier-to-interference ratio (C/I),which results in a large capacity (e.g., a hundred or more voice usersmay be serviced by 1.25 MHz on the forward link). For the Walsh covermultiplexing scheme, the channel estimates can be more accurate forfixed applications and where directional antennas are deployed. Thisallows for more accurate equalization of the transmission channel tomaintain orthogonality of the Walsh covered data.

For mobile application, soft handoff such as that employed in IS-95 CDMAsystems can be used to transfer a mobile user terminal from one basestation to another. To achieve soft handoff, a base station controllercan request that all base stations in soft handoff send the user'spackets on a common circuit or common OFDM tones. The base stations canbe coordinated to achieve this. Alternatively, the base stations in softhandoff can transmit packets on circuits available to them. The userterminal can digitize the received signal and process the samples torecover the packets transmitted by the base stations. The processing ofthe transmissions from the base stations can be performed usingdifferent parameters (e.g., different PN offsets, different circuits).The user terminal can also combine the processed results (similar tothat performed by a rake receiver) to generate a combined result havingimproved performance.

The above multiplexing, transmission, and signaling schemes have beendescribed for the forward link transmission from the base station to theuser terminal. At least some of the concepts described herein can beapplied for the reverse link transmission from the user terminal to thebase station.

As shown above, the transmitter unit and receiver unit are eachimplemented with various processing units that include various types ofdata processor, encoders, IFFTs, FFTs, demultiplexers, combiners, and soon. These processing units can be implemented in various manners such asan application specific integrated circuit (ASIC), a digital signalprocessor, a microcontroller, a microprocessor, or other electroniccircuits designed to perform the functions described herein. Also, theprocessing units can be implemented with a general-purpose processor ora specially designed processor operated to execute instruction codesthat achieve the functions described herein. Thus, the processing unitsdescribed herein can be implemented using hardware, software, or acombination thereof.

Measuring and Reporting Channel State Information In A MIMO System

As discussed above, full CSI may include sufficient characterization ofthe propagation path (i.e., amplitude and phase) between all pairs oftransmit and receive antennas for each sub-channel. CSI may also includethe information of the relative levels of interference and noise in eachsub-channel, that is known as C/I information. The CSI may be embodiedin a set of matrices of complex gain values that are descriptive of theconditions of the transmission paths from the transmit antennas to thereceive antennas, as described below. With CSI, the transmitter unitpre-conditions the data prior to transmission to receiver unit.

As discussed above, the transmitter unit can thus select a modulationalphabet (i.e., signal constellation) for each of the eigenvectors basedon the C/I that is given by the eigenvalue. Provided that the channelconditions do not change appreciably in the interval between the timethe CSI is measured at the receiver and reported and used toprecondition the transmission at the transmitter, the performance of thecommunications system will be equivalent to that of a set of independentAWGN channels with known C/I's.

Such a system is illustrated in FIG. 11. At step 141, the transmitterunit 140 converts data into multiple data sub-channels. Different QAMconstellations are employed, depending upon the SNR of the mode andsub-channel. The data for each sub-channel is preconditioned by theeigenmode matrix for that sub-channel. At step 142, the preconditioneddata for a particular antenna undergoes an inverse-Fast FourierTransform (IFFT) operation to produce a time-domain signal. At step 143,a cyclic extension or a cyclic prefix is appended to the time-domainsignal in order to maintain orthogonality among the OFDM sub-channels inthe presence of time-dispersion in the propagation channel. One extendedsymbol value is generated far each OFDM sub-channel and will be referredto hereafter as an OFDM symbol. At step 144, the OFDM symbols aretransmitted from the multiple transmit antennas.

Multiple antennas at a receiver unit 145 receive signals at step 146. Atstep 147, the received signals undergo a Discrete Fourier Transform(DFT) operation to channelize the received signals. At step 148, thedata from each subchannel over all of the receive antennas is processed.At this processing step, information regarding channel characteristicsis extracted from the data, and converted into a more compressed format.One compression technique is the use of the conjugate channel responseand the eigenmode matrix to reduce the amount of information needed todescribe channel characteristics. At step 149, a message containing thecompressed channel state information is transmitted from the receiverunit 145 to the transmitter unit 140, which will then be used toprecondition further transmissions.

To facilitate the derivation of the CSI, the transmit waveform is madeup of known pilot symbols for an initial preamble. The pilot waveformsfor different transmit antennas comprise disjoint sets of OFDMsubchannels as illustrated for the case when N_(t)=4 in FIG. 12.

With OFDM modulation, the propagation channel is divided into L parallelsub-channels. In order to determine the CSI quickly, an initial preambleconsisting entirely of known symbols is transmitted. In order toefficiently distinguish the differing channel responses of the differenttransmit-receive antenna patterns, the pilot signals are assigneddisjoint subsets of sub-channels. FIG. 12 is a diagram of an exemplaryOFDM pilot structure composed of disjoint sub-channel subsets. Asub-channel set composed of sub-channels {0, 1, 2, . . . , 2 ^(n)−1} isdecomposed into four disjoint sub-channel subsets A={0, 4, 8, . . . ,2^(n)−4}, B={1, 5, 9, . . . , 2^(n)−3}, C={2, 6, 10, . . . , 2^(n)−2}and D={3, 7, 11, . . . , 2^(n)−1}. Sub-channel subset A 150 istransmitted on transmit antenna T×1 151, sub-channel subset B 152 istransmitted on transmit antenna T×2 153. sub-channel subset C 154 istransmitted on transmit antenna T×3 155. and sub-channel subset D 156 istransmitted on transmit antenna T×4 157. Generally, each transmitantenna transmits on every N^(th) sub-channel across the channel so thatall sub-channels are disjoint between transmit antennas. Known pilotsymbols can be transmitted on all sub-channels in a sub-channel subset.The minimum spacing between the sub-channels used by a particulartransmit antenna is a function of the channel parameters. If the channelresponse has a large delay spread, then a close spacing may benecessary. If the number of antennas is large enough that the requiredspacing may not be achieved for all users with a single OFDM symbol,then a number of consecutive OFDM symbols may be employed, with eachantenna assigned a disjoint subset of sub-channels on one or more of themultiple pilot symbols.

From each transmit antenna at a transmitter unit, the receiver unitreceives pilot symbols on disjoint sub-channels and makes determinationsas to channel characteristics of the disjoint sub-channels. As discussedpreviously, the receiver unit may have one or more receive antennas.Suppose x ={x_(i), i=l, . . . , K}are the pilot symbol values that areto be transmitted on K pilot ub-channels for a single transmit antenna.The receiver unit will receive the values y_(ij)=h_(ij)x_(i)+n_(ij),wherein h_(ij) is the complex channel response for the i^(th) pilotsub-channel received at the j^(th) receive antenna, and n_(ij) is noise.From this relationship, the receiver unit can determine noisy estimatesof the channel response of K sub-channels of a single transmit antenna.These noisy estimates may be used to derive estimates for allsub-channels of the propagation channel through a number of differentmethods, such as simple interpolation to more complex estimation using apriori information on the channel dispersion and noise level. Theestimates may be improved by transmitting pilot symbols over consecutiveOFDM symbols and then averaging the estimates for each consecutive OFDMsymbol.

Estimates are generated at each receive antenna for each transmitantenna broadcasting pilot symbols. The CSI for the complete propagationchannel can be represented by the set of channel response matrices{H_(i), i=1, 2, . . . , 2^(n)}, where matrix H_(i) is associated withthe i^(th) sub-channel, and the elements of each matrix H_(i) are{h_(ijk), j=1,. . . ,N_(r),k=1,. . . ,N_(t)}, the complex channelresponse values for each of the N₉₆ transmit and N _(r) receiveantennas.

The use of disjoint sub-channel subsets can further be applied in asystem wherein multiple links, e.g., a propagation channel from atransmitter unit to one or more receiver units, are located in closeproximity. In a system where a base station transmits signals accordingto sectors, the transmission area of a sector can overlap thetransmission area of another sector. In an ideal base station, transmitantennas in each sector transmit signals in a direction that iscompletely disjoint from the directions assigned to the transmitantennas of the other sectors. Unfortunately, overlapping areas exist inmost sectored base stations. Using this embodiment of the invention, alltransmit antennas of a base station are assigned disjoint subsets ofsub-channels to avoid interference between the sectors of that basestation. Similarly, neighboring base stations may also be the cause ofsignificant interference, and disjoint sets of sub-channels may beassigned among base stations.

In general, the computation of the channel response can be made forevery link that is assigned a disjoint sub-channel subset, in the samemanner as the response is computed for the principle link. However, areduced amount of CSI from these interfering links may be reported tothe transmitter unit. For example, information as to the average totalinterference level of neighboring links can be transmitted and used todetermine the supportable data rate of the principle link. If severalinterfering links dominate the average total interference level, thenthe interference information of these links may be reported individuallyto the system in order to determine a more efficient grouping ofsub-channels in each disjoint sub-channel subset.

Other CSI information that can be conveyed to the transmitter unit isthe total measured power in sub-channels not assigned to the principallink. The total measured power of sub-channels assigned to neighboringlinks gives an estimate of the total interference plus noise power. Ifseveral OFDM symbols are used as the pilot symbol, then the meanmeasured channel response and the actual received signal values may beused to make a direct estimate of the total noise in a givensub-channel.

In general, the assignment of sub-channels for a network of basestations should follow a pattern of “frequency-reuse,” wherein the samesub-channels are used only when the links are sufficiently separated bydistance. If a large number of links are interfering with each other,then the number of OFDM sub-channels may be inadequate to allow theassignment of sub-channels for every pilot OFDM symbol. In thiscircumstance, transmit antennas may be assigned sub-channels for everyP-th pilot symbol, where P is an integer value greater than (1).

In another embodiment of the invention, the OFDM scheme is designed tocreate OFDM symbol values that minimize or eliminate interferencebetween transmit antennas that use either identical sub-channels ordisjoint sub-channels. An orthogonal code, such as Walsh coding, can beused to transform Q pilot signals into Q orthogonal signalsrepresentative of the pilot signals. In the case where a Walsh code isused, the number of pilot signals would be a power of two. The use oforthogonal codes can be used together with the previously discusseddisjoint sub-channel subsets in order to reduce interference fromneighboring links. For example, in a 4×4 MIMO system with a systembandwidth of an proximately 1 MHz, assume that 256 OFDM sub-channels areto be used. If the multipath is limited to ten microseconds, thedisjoint sub-channels carrying pilot symbols should be spacedapproximately 50 kHz apart or closer. Each sub-channel is approximately4 kHz wide so that a spacing of twelve sub-channels is 48 kHz wide. Ifthe OFDM sub-channels are divided into twelve sets of twentysub-channels each, sixteen are left unused. Two consecutive OFDM symbolsare used as a pilot signal, and orthogonal coding on these two symbolsis employed. Hence, there are twenty-four different orthogonal pilotassignments. These twenty-four orthogonal pilots are assigned todifferent transmit antennas and links to minimize interference.

In another embodiment of the invention, a large number of periodic OFDMsymbols can be used as pilot data. The number of OFDM symbols must belarge enough so that accurate measurements of interference levels from alarge number of different transmit antennas can be made. These averageinterference levels would be used to set up system-wide constraints onsimultaneous transmissions from various sites, i.e., an adaptiveblanking scheme to allow all users nearly equivalent performance.

In an alternate embodiment of the invention, the CSI of a MIMOpropagation channel can be determined and transmitted for a MIMO systemthat does not utilize OFDM symbols as pilot signals. Instead, aMaximal-Length Shift Register sequence (m-sequence) can be used to soundthe propagation channel. An m-sequence is the output of a shift registerwith feedback. M-sequences have desirable autocorrelation properties,including the property that correlation over a full period of thesequence with any non-zero circular shift of the sequence yields thevalue −1, wherein the sequence values are +/−1. Hence, the correlationat zero shift is R, wherein R is the length of the sequence. In order tomaintain desirable properties such as correlation in the presence ofmultipath, a portion of the sequence equal to the delay spread of thechannel must be repeated.

For example, if it is known that the channel multipath is limited tosome time T_(m), and the length of the pilot sequence is at least RT_(m), then R different shifts of the same m-sequence may be used withonly minimal mutual interference. These R different shifts are assignedto different transmit antennas of a base station and other base stationsthat are the cause of major interference.

Links in the MIMO system that are distantly separated can be assigneddifferent m-sequences. The cross-correlation properties of differentm-sequences do not exhibit the minimal correlation properties of asingle sequence and its shifts, but different m-sequences behave more orless like random sequences and provide an average correlation level of√{square root over (R)} where R is the sequence length. This averagecorrelation level is generally adequate for use in a MIMO system,because of the separation between the links.

A shift resister with feedback generates all possible m-sequences, sothat sequences are merely shifted versions of a signal code word oflength R=2^(m)−1, where m is a positive integer value. Hence, a limitednumber of different binary m-sequences exist. In order to avoid reuse ofthe same m-sequence in an area where significant interference mayresult, filtered versions of longer m-sequences can be used. A filteredversion of an m-sequence is no longer binary, but will still display thesame basic correlation properties.

For example, suppose that the pilot sequence is to be transmitted at a 1MHz rate, and that the multipath is limited to ten microseconds. Assumethat a base station has three sectors, wherein four transmit antennasare assigned to each sector for a total of twelve transmit antennas persite. if a length 127 m-sequence is employed, then twelve differentshifts of the sequence may be assigned to the antennas of a single basestation, with relative shifts of ten samples each. The total length ofthe transmitted pilot is then 137 microseconds, which is a full periodof the sequence plus ten additional samples to accommodate the multipathspread. Then different base stations can be assigned differentm-sequences, with m-sequences repeated in a code reuse pattern designedto minimize the effects of interference from the same m-sequence.

The embodiments of the invention discussed herein have been directed tothe design and transmission of pilot signals that will allow a personskilled in the art to derive characteristics of the propagation channeland to report such characteristics to the transmission site. However,the full CSI is a large amount of information and also highly redundant.Many methods are available for compressing the amount of CSI informationto be transmitted. One method discussed previously is the use of theHermatian matrix H*H, wherein H is the channel response as determined atthe receiver unit. The Hermation matrix H*H can be reported to thetransmitter unit and be used to precondition transmissions. Due to theproperties of Hermitian matrices, only half of the matrix elements needto be transmitted, such as the complex lower triangular portion of thematrix H*H, and the real-valued diagonal. Additional efficiencies arerealized if the number of receive antennas is larger than the number oftransmit antennas. Another method to reduce the amount of informationtransmitted to the transmitter unit on the reverse link is to reportonly a subset of the channel response matrices Hi to the transmitterunit, from which the unreported channel response matrices can bedetermined Through interpolation schemes. In another method, afunctional representation of the channel response across thesub-channels may be derived for each transmit/receive antenna pair,e.g., a polynomial function representative of the channel response canbe generated. The coefficients of the polynomial function are thentransmitted to the transmitter unit.

As an alternative to these methods for compressing CSI information, oneembodiment of the invention is directed to the transmission of atime-domain representation of the channel response, which is the channelimpulse response, If a time-domain representation of the channelresponse is simple, as in cases where there are only two or threemultipath components, an inverse FFT can be performed for upon the setof channel frequency responses. The inverse FFT operation can beperformed for each link between a transmit/receive antenna pair. Theresulting channel impulse responses are then translated into a set ofamplitudes and delays that are reported to the transmitter.

As discussed previously, there is a cost associated with thetransmission of CSI in the reverse link, which is reduced when the aboveembodiments of the invention are implemented in the MIMO system. Anothermethod for reducing the cost is to select users according to the shortterm average of their CSI requirements. The CSI requirements change asthe channel fades, so improved efficiency on the reverse link isachieved if users estimate the quantity of CSI required, and inform thebase station at intervals that may be periodic or aperiodic, dependingon the rate of change of the propagation channel observed by the user.The base station may then include this factor in scheduling the use ofthe forward and reverse links. Scheduling can be arranged so that usersassociated with slowly changing propagation channels report lessfrequently than users associated with quickly changing propagationchannels. The base station can also arrange the scheduling to take intoaccount factors such as the number of system users and fairness.

In another aspect of this embodiment of the invention, a time intervalcan be assigned so that CSI updates in a long transmission period can beadjusted according to the actual changes in the propagation channel.Changes in the propagation channel can be monitored at the receivingsite in one of a number of possible ways. For example, the differencebetween the soft decision on the symbols and the closest QAMconstellation value can be determined and used as a criterion, or therelative sizes of decoder metrics can also be used. When the quality ofa given criterion falls below a predetermined threshold, an update tothe CSI is reported to the transmitter unit.

The overall multipath power-delay profile of a link changes very slowlybecause the average power observed at various delays remains constant,even though channel fading may occur frequently. Hence, the amount ofCSI required to characterize a link can vary substantially from link tolink. To optimize performance, the coding of the CSI is tailored to thespecific link requirements. If the CSI is sent in frequency-domain form,i.e., a set of channel response matrices which are to be interpolated.then links with little multipath require only a small set of channelresponse matrices.

The foregoing description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without the use of theinventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

1. A method of transmitting in a multi-antenna communication system,comprising: modulating a plurality of signals to be transmitted fromeach antenna of a plurality of antennas with at least one subband of adifferent one of a plurality of groups of subbands from availablesubbands, wherein each of the plurality of groups of subbands includes adifferent subset of the available subbands and wherein the subbands of afirst group are noncontiguous; and transmitting at least some of theplurality of signals substantially simultaneously from different ones ofthe plurality of antennas.
 2. The method of claim 1, wherein each of theplurality of groups of subbands includes a same number of subbands. 3.The method of claim 1, wherein the subbands in each of the plurality ofgroups of subbands are uniformly distributed across the availablesubbands.
 4. The method of claim 1, further comprising orthogonallycoding a pilot symbol that is to be transmitted from at least one of theplurality of antennas.
 5. The method of claim 4, wherein the pilotsymbol is orthogonally coded with a Walsh code sequence.
 6. The methodof claim 4, wherein the pilot symbol is encoded with a shiftedMaximal-Length Shift Register sequence (m-sequence).
 7. The method ofclaim 1, wherein the multi-antenna communication system utilizesorthogonal frequency division multiplexing (OFDM).
 8. A base stationcomprising: a plurality of transmit antennas; a processor configured toinstruct a plurality of pilot symbols to be transmitted from theplurality of transmit antennas with one subset of a plurality ofsub-channel subsets from available sub-channels, wherein eachsub-channel subset comprises a plurality of sub-channels that arenoncontiguous and includes different ones of the available sub-channels;and a plurality of modulators, each modulator coupled to the processorand to at least one of the plurality of transmit antennas, configured tomodulate the plurality of pilot symbols with appropriate sub-channels ofthe one subset of the plurality of sub-channel subsets.
 9. The basestation of claim 8, wherein the plurality of pilot symbols comprises aplurality of orthogonal pilot symbols.
 10. The base station of claim 8,wherein the plurality of pilot symbols comprises a plurality of periodicOFDM symbols.
 11. The base station of claim 8, wherein the processorcodes the plurality of pilot symbols with a plurality of shiftedMaximal-Length Shift Register sequences (m-sequences).
 12. The basestation of claim 8, wherein each of the plurality of sub-channel subsetsincludes a same number of available sub-channels.
 13. The base stationof claim 8, wherein the plurality of sub-channels in each of theplurality of sub-channel subsets is are uniformly distributed across theplurality of sub-channels.
 14. An apparatus in a multi-antennacommunication system, comprising: means for modulating a plurality ofsignals to be transmitted from each antenna of a plurality of antennaswith at least one subband of a different one of a plurality of groups ofsubbands from available subbands, wherein each of the plurality ofgroups of subbands includes a different subset of the available subbandsand wherein the subbands of a first group are noncontiguous; and meansfor transmitting at least some of the plurality of signals substantiallysimultaneously from different ones of the plurality of antennas.
 15. Theapparatus of claim 14, wherein each of the plurality of groups ofsubbands includes a same number of subbands.
 16. The apparatus of claim14, wherein the subbands in each of the plurality of groups of subbandsare uniformly distributed across the available subbands.
 17. Theapparatus of claim 14, further comprising means for orthogonally codinga pilot symbol that is to be transmitted from at least one of theplurality of antennas.
 18. The apparatus of claim 17, wherein the pilotsymbol is encoded with a Walsh code sequence.
 19. The apparatus of claim17, wherein the pilot symbol is encoded with a shifted Maximal-LengthShift Register sequence (in-sequence).
 20. The apparatus of claim 17,wherein the multi-antenna communication system utilizes orthogonalfrequency division multiplexing (OFDM).
 21. A computer-program productfor transmitting in a multi-antenna communication system comprising acomputer readable medium executable using one or more processors havinginstructions thereon, the instructions comprising: instructions formodulating a plurality of signals to be transmitted from each antenna ofa plurality of antennas with at least one subband of a different one ofa plurality of groups of subbands from available subbands, wherein eachof the plurality of groups of subbands includes a different subset ofthe available subbands and wherein the subbands of a first group arenoncontiguous; and instructions for transmitting at least some of theplurality of signals substantially simultaneously from different ones ofthe plurality of antennas.
 22. The computer-program product of claim 21,wherein each of the plurality of groups of subbands includes a samenumber of subbands.
 23. The computer-program product of claim 21,wherein the subbands in each of the plurality of groups of subbands areuniformly distributed across the available subbands.
 24. Thecomputer-program product of claim 21, further comprising instructionsfor orthogonally coding a pilot symbol that is to be transmitted from atleast one of the plurality of antennas.
 25. The computer-program productof claim 24, further comprising instructions for orthogonally coding thepilot symbol with a Walsh code sequence.
 26. The computer-programproduct of claim 24, further comprising instructions for coding thepilot symbol with a shifted Maximal-Length Shift Register sequence(m-sequence).
 27. The computer-program product of claim 24, wherein themulti-antenna communication system utilizes orthogonal frequencydivision multiplexing (OFDM).